April 20, 2017 | Author: benjamin_wang_4 | Category: N/A
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HSC Biology Topic 2
BLUEPRINT of LIFE What is this topic about? To keep it as simple as possible, (K.I.S.S.) this topic involves the study of: 1. EVOLUTION 2. MENDELIAN GENETICS 3. CHROMOSOMES & DNA STRUCTURE 4. MOLECULAR GENETICS & MUTATION 5. REPRODUCTIVE TECHNOLOGIES & GENETIC DIVERSITY
but first, an introduction... Evolution
Chromosomes & DNA
In earlier topics you have already been introduced to the concept of Biological Evolution, and the mechanism of “Natural Selection”. Now you will study in more detail the evidence for evolution and Theory of Evolution.
You will next move on to study things that Darwin & Mendel would have been delighted to have known... the cellular and molecular basis of Genetics and Evolution. You will see the link between inheritance, cell division, chromosomes and the basis of it all... the DNA molecule and the Genetic Code. Simulated Photograph of Human Chromosomes
Part of a DNA molecule
Photo: Sasan Saidi
Genetics The science of inheritance was discovered about 150 years ago by a monk living in an obscure monastery in central Europe. In this topic you will study the simple patterns of inheritance that he discovered.
A section of DNA carries a code to specify one feature of the organism; = a gene
In the section on Molecular Genetics you will learn how the DNA acts to control all the characteristics of each organism (the “phenotype”) and how Mutations can occur to alter the code and produce new characteristics for evolution to work on. In the final section you will look briefly at where modern Biology is heading into Reproductive Technologies and Genetic Engineering. (This controversial area can be studied more thoroughly in one of the later Option topics)
Gregor Mendel (1822-1884) experimented with garden peas for 12 years. You will study his results and the reasons for his success as an experimenter. HSC Biology Topic 2 copyright © 2005-2007
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CONCEPT DIAGRAM (“Mind Map”) OF TOPIC
Some students find that memorizing the OUTLINE of a topic helps them learn and remember the concepts and important facts. As you proceed through the topic, come back to this page regularly to see how each bit fits the whole. At the end of the notes you will find a blank version of this “Mind Map” to practise on.
•Palaeontology •Biogeography •Comparative Embryology •Comparative Anatomy • Biochemistry
The EVIDENCE for Evolution
Natural Selection, Survival of the Fittest
Divergent & Convergent Evolution
Alleles & genes. Homozygous, heterozygous. Genotypes & phenotypes
His results and explanations
Mendel’s experiments, and the reasons for his sucess
Dominant-Recessive inheritance. Monohybrid crosses DNA structure
Mendelian Genetics
Chromosomes, genes & DNA
Chromosomes & DNA
THE BLUEPRINT OF LIFE
Non-Mendelian Inheritance. Sex-linkage & Codominance
Reproductive Technologies & Genetic Diversity
Meiosis & gamete formation
Sources of variation
Some history of our understanding
Molecular Genetics & Mutations
Genetic Engineering and “Transgenic species”
Effects on genetic diversity HSC Biology Topic 2 copyright © 2005-2007
Punnett squares
The THEORY of EVOLUTION
Evolution
Artificial insemination and pollination. Cloning
Pedigrees Family trees
Mutation & mutagens
Punctuated Equilibrium keep it simple science
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DNA Replication
DNA Transcription & Translation
The link to evolution
The link to phenotypes
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1. EVOLUTION The Evidence for Evolution
Fossils of “Transitional Forms” Only a tiny fraction of all the zillions of organisms that have ever lived have ended up being fossilized. The fossil record is, therefore, incomplete and a rather patchy record of evolution.
There is overwhelming evidence that life on Earth has undergone a sequence of changes over the past 3.5 billion years or so. Here, briefly (K.I.S.S. Principle) are the main sources of that evidence:
Palaeontology
Despite this “patchiness”, there are some fossils that have given us a glimpse of the change that occurred when one type of life was evolving into another. Such fossils are called “Transitional Forms”
The Study of Fossils The fossil evidence is undoubtably our most important, direct evidence showing that life on Earth has changed. The fossils not only show that life was once different, but that the changes follow a pattern or sequence... ...this is evolution. ANCIENT TIMES
MODERN TIMES
Less complexity Less variety Less like present-day life
More complex types Greater variety More and more resembling modern life
Perhaps the most famous is Archaeopteryx. (“Archae”= ancient, “pteryx”= wing) When this fossil was first discovered, its skeleton was clearly that of a small dinosaur. Only later was it noticed that the faint outlines around the fossil were the impressions of feathers. This was a dinosaur-bird! Reconstruction of Archaeopteryx
Bony tail covered in feathers
How do we know this to be fact? Not only do we have many fossils of extinct organisms, but we can place them in time sequence to see the patterns. Sediments are laid down on top of the previous layer, so the lower layers are older. Rocks and fossils location 1
Rocks and fossils location 2
Bony jaw with teeth
youngest fossil
Clawed wings
fo fr ssi to om ls c an on orr ot e ela he pla te r ce
It is very unlikely that Archaeopteryx could fly like a modern bird. It may have climbed trees & glided down on insect & lizard prey
Oldest fossil
Transitional fossils are important evidence that each new type of life that “appears” in the fossil record did in fact evolve from a previous ancestor. Scientists have discovered transitional fossils showing:
Initially, scientists could only place fossils in relative time order by correlating the sequences, as suggested in the diagram above.
• reptiles evolving to become mammals • ferns evolving to become cone-bearing plants • lobe-finned fish evolving to become amphibians
Now we can also place absolute times on many fossils by “Radiometric Dating”... the determination of the actual age of things by measuring the residual radioactivity of certain, naturally-occurring radio-isotopes in the rocks. (To revise this more thoroughly, see Preliminary Topic 3 “Life on Earth” page 4) HSC Biology Topic 2 copyright © 2005-2007
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...and many more, including fossils of our own probable ancestors, who were very ape-like, but undoubtably human-like too! 3
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Biogeography (the study of how life is distributed)
Comparative Embryology
When Charles Darwin travelled around the world on board HMS Beagle in the 1830’s, it was the distribution of different types of life in different places that first gave him the idea for Evolution.
If the embryos of various vertebrate animals are compared, it is found that they are remarkably similar, even though they later develop into quite different animals. Some Vertebrate Embryos...
He noticed that each continent had its own characteristic biota, often showing similar features although often not closely related to each other. For example, each continent has characteristic grazing mammals: African Antelope
Luis Rock OZ Roo
Photos: Diana
...and what they develop into
Why should they be so similar when they grow up to be very different animals? Why should (for example) a human embryo have a tail and gill arches like a fish?
North American Bison
Steve Clayton
All these animals are relatively large, fast-running, with flat grinding teeth, and all lead pretty much the same lifestyle. So why is there not just one type of large grazer in the world, living on every continent?
Evolution explains this as the result of all these animals evolving from a common ancestor which had an embryo like this.
Comparative Anatomy
Evolution explains why. On each separate continent different animals have evolved to fill the “large grazing herbivore” role, often beginning that evolution from an ancestor quite different to that on other continents.
When the body structures of different organisms are compared, it is often obvious that they share common features, even though those body parts might be used in different ways.
Another aspect of Biogeography was seen by Darwin on a much smaller scale in the Galapagos Islands. He discovered that the islands were populated by many different species of small birds... finches. Although all were obviously related to each other, each separate island had its own particular species.
The classic example of this is the Pentadactyl Limb structure of the vertebrate animals. Some Vertebrate Forelimbs (Arms) (Not to same scale)
Ancestral Finch
1 bone in upper arm
Cat Leg
2 bones in forearm Island 1
Island 2
Island 3
5 digits
Why? The explanation is: • the islands were first colonized by one type of finch, which spread to all of the islands in the group. • on each separate island conditions were different (eg different foods available) so each population evolved in a different way, into a different species.
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Human Arm
Bat Wing
Lizard Leg
Each limb is used in very different ways by animals that have very different lifestyles, yet all have exactly the same basic arrangement of bones. Why? Evolution explains this as the result of all these animals evolving from a common ancestor which had this bone structure.
BIOGEOGRAPHY doesn’t prove Evolution has occurred, but many facts about the distribution of organisms are best explained by Evolution HSC Biology Topic 2 copyright © 2005-2007
Whale Flipper
Once again, these are FACTS that are consistent with, and easily explained by, the concept of EVOLUTION. 4
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(“Penta”= 5, “dactyl”=fingers)
Sth American Llama
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Biochemistry
Selective Breeding
All the life-forms on Earth have remarkable similarities at the molecular level. All organisms have basically the same:
Humans have “domesticated” many plants and animals such as cattle, sheep, chickens, dogs, rice, wheat, tomatoes, and so on. Over thousands of years, human farmers and breeders have had control over the breeding of these organisms. Often a farmer or animal keeper will choose very carefully which individuals will be allowed to breed, always selecting the best ram to breed with the ewes, or seeds from the best plants to sow for next year’s crop.
• genetic code in their DNA • enzymes used for basic metabolic processes, such as cellular respiration • structural chemicals in their basic cell components, such as the phospholipids in cell membranes.
Side-ttrack
WHY SHOULD ALL ORGANISMS HAVE THE SAME STRUCTURES AND CODES IN THEIR DNA?
The result is that these species have changed. All varieties of domestic dog, for example, belong to the same species, and all are believed to be descended from the wolf which may have been domesticated as the first “puppy” about 12,000-15,000 years ago.
Once again, this doesn’t prove that evolution occurred, but it is consistent with the idea that life on Earth has evolved from common ancestors who had these biochemical features.
How New Technologies are Changing Our Ideas About Evolutionary Relationships
Photos by Diana
“SPIKE”
The new technology of analysing the sequence of DNA molecules is changing our ideas about the evolutionary relationships between living things. For example, the evolutionary relationship between LIZARDS, CROCODILES and BIRDS.
Selective Breeding proves that a species can be changed. Humans can do it artificially, in the wild it happens by “Natural Selection”
Traditionally crocodiles & lizards are classified as “Reptiles” and considered a separate class to the “Birds”. It was always thought that the lizards and crocodiles were closer in an evolutionary sense, and shared a common ancestor closer together than either was to the birds.
The preceeding pages have outlined the facts & evidence for what has happened.
Based on structural similarities & differences
CLOSELY RELATED
More Distant
This relationship has been overturned by DNA analysis. DNA sequencing shows that birds and crocodiles are more closely related to each other than either is to the lizards.
Next we look at the how & why... The Theory of Evolution, by “Natural Selection”
Based on DNA analysis
CLOSELY RELATED
More Distant
Stand by for more revelations as DNA analysis reveals more! HSC Biology Topic 2 copyright © 2005-2007
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Darwin & Wallace
More on “Natural Selection”
In the 1830’s a young naturalist, Charles Darwin (1809-1882), voyaged around the world on the naval survey ship HMS Beagle. He became convinced that living things had changed, and spent the next 20 years researching, gathering evidence and developing the “how” of a theory to explain evolution.
The following is revision material from Preliminary Topic 4 and needs to be understood... “Natural Selection” refers to the way that the conditions of nature constantly select who survives and who dies. If survival depends on speed to run from a predator, then the faster individuals survive and the slower ones get eaten. In a drought those plants with slightly thicker cuticle on their leaves conserve water and survive, while others die. Any characteristic might be a help to survive under different conditions... • a better immune system helps, when diseases strike... • more inquisitive behaviour might find more food... • more timid behaviour might avoid dangers... • brighter coloured flowers might attract pollinators... • larger body size might deter predators... • smaller body size might allow more food gathering on the ends of small branches of trees.
Meanwhile, another Englishman, Alfred Wallace (18231913) came up with exactly the same idea after his journeys in the 1840’s and 1850’s. We often give Darwin most of the credit for this major scientific theory, but Wallace’s contribution should not be forgotten.
It is difficult to predict exactly which characteristic, or combination of characteristics, might help survival... it depends on what happens in the environment: HOW NATURAL SELECTION WORKS
In 1859 they jointly published an outline of their theory, and Darwin followed up with his detailed book “Origin of Species” about 2 years later.
A population of a species with a lot of “variations”
The Darwin-Wallace Theory of Evolution You need to learn the details of this theory. This is best done as a series of numbered points:-
hairy
long legs big ears long neck dark colour
squat body
Then, the climate changes... winters get colder
1. All organisms produce more offspring than can survive. Plants scatter thousands of seeds and maybe only one ever makes it to plant maturity. Oysters produce a million eggs at a time, but only 1 or 2 survive. In all species, the breeding rate is higher than the survival rate.
Many die in the harsh winters
hairy survives The survivors pass on their characteristics. Over generations most of the population become squat and hairy.
2. In every species there is variation among the individuals. Each animal or plant is slightly different to the other members of the species. The differences may be slight, and might not be easily visible, but they’re important!
Squat survives
Survivors breed
If you put points 1 & 2 together, you can see that survival is uncertain, AND the chances of survival are not equal for each individual.
The species has changed... it has evolved by adapting to changes in its environment.
3. Nature selects which individuals survive. This is called “Natural Selection”, and it is explained in more detail soon.
It may seem as if “Evolution” is an intelligent force which directs organisms in a an appropriate direction. In our fictitious population of animals, the climate became colder and it would seem that some evolutionary force caused an appropriate change in the population... they became squat & hairy to conserve body heat better.
4. Those best suited to their environment survive, and reproduce, and pass on their characteristics to their offspring. This is called “Survival of the Fittest”, and it will be discussed in more detail later.
But of course “squat” and “hairy” were already in the population. Their type simply became more common, and finally predominant, because they survived in greater numbers and reproduced to pass on their characteristics to their offspring.
5. Over generations, the species changes (Evolves) as each generation is slightly different than before, because only some, selected individuals have bred and passed on their characteristics. HSC Biology Topic 2 copyright © 2005-2007
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Evolution is not intelligent. Nature selects the survivors from the different “types” available. Survivors breed... if you’re dead, you can’t reproduce!
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More on “Survival of the Fittest”
Effects of Changing Conditions
“Survival” does not just mean survival. The biggest toughest animal in the herd, who scares predators away, gets to eat most of the food and lives to a ripe old age, is a complete failure if it does not reproduce!
Environments can change in many different ways. For example, we know that ice-ages come and go, and climates change as the continents gradually “drift” to new locations. Any change in an environment will result in Natural Selection picking out different characteristics for survival and the species will evolve in line with the environmental change (OR, if the change is sudden and drastic, there will be fewer survivors every generation and the species will decline into extinction!)
In evolutionary terms “Survival” means to survive and reproduce. Reproduction is the true measure of “survival”. An animal which does not live long, but manages to produce many, viable offspring is an evolutionary success!
The following case studies serve to make this point, and are also examples of observed “micro-evolution” providing further evidence that Evolution by Natural Selection can and does occur.
“Fittest” refers to those individuals with a combination of characteristics best suited to their environment. It doesn’t necessarily mean biggest, fastest, strongest... those best suited to some environments might be the smallest, sneakiest, most cautious types.
A Change in the Physical Environment A classic example is the change in the population of the English Pepper Moth which has been studied and documented over the past few hundred years.
... or the most charming and socially acceptable
This moth always rests during the day on tree trunks, which in natural forests, are mostly covered in motley-patterned lichens. Under these conditions the light “peppery” moths are the most common form, although occasional black moths occur. In unpolluted forests, the lighter-coloured moths are camouflaged st”
tte
t igh
L
ths
mo
“fi are
Predators spot the black moths more easily
The Importance of Variation It is vital for the on-going survival of a species that it has variation among the individuals of the population.
Lighter-coloured moths survive and breed in greater numbers
Population ratio
During the coal-burning phase of the Industrial Revolution many forests were damaged by pollution. The lichens were killed and tree trunks blackened with soot.
When changes occur in the environment, a species with a lot of variation has more chance to survive, because out of all the different “types” there is a good chance that at least some will survive to breed and continue the species.
It was observed that the Pepper Moth population changed in the proportion of peppery to black types. In polluted forests, the black moths are camouflaged
m
ot
hs
ar e
“fi
tte
st
”
A species with little variation within it, might have no survivors from an environmental change, and become extinct.
Black moths survive and breed in greater numbers
HSC Biology Topic 2 copyright © 2005-2007
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Bl a
ck
What constitutes an environmental change? It could be... • a change of climate • a new disease, predator or competitor in the ecosystem • a change in availability of a food resource ...or any other biotic or abiotic factor.
Predators spot the light moths more easily
Population ratio
Now that industrial pollution has been stopped, the moths have evolved back to being mostly of the peppery type. 7
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A Change in the Chemical Conditions of the Environment Another example of “micro-evolution” was observed when DDT insecticide began to be used against a variety of insects, such as disease-carrying mosquitoes or crop-eating pests.
Divergent Evolution
Initially, the chemical was a huge success, decimating the insect populations. But then Natural Selection did its thing...
Ancestral Finch
(“Diverge” = move apart) One of the aspects of evolution to be aware of is the importance of Isolation. Rember Darwin’s finches on the Galapagos Islands?
More
Among the millions of insects in each population there was variation. A few individuals had a natural resistance to the DDT and they survived and reproduced and passed on their resistance to their offspring. Over many generations the non-resistant types were killed, and resistant types kept surviving and breeding until almost the entire population was resistant. DDT was no longer useful for killing insects. (Just as well too, because it was later discovered to have some negative environmental impacts on other species)
Island 1
On each island conditions were different... different foods were available, different conditions of water supply, shelter, nesting sites, predators and so on. Natural selection caused each population to evolve along a different path until they became different species... they had diverged!
Competition for Resources In a previous topic it was pointed out that competition between 2 species for the same resources usually results in a “winner” and a “loser”... the loser becoming extinct, at least within the area of competition.
Now, even if they mixed together again, they could not interbreed because their mating rituals, sperm & egg cells and DNA have changed to become incompatible. They are forever separate. This is how we think all species have arisen from common ancestors over millions of years.
Population Size (ABUNDANCE)
A TYPICAL COMPETITION GRAPH
New competitor enters ecosystem
W
IN
N
ER
LO
Convergent Evolution (“Converge”=come together) If totally different organisms live in the same kind of environment and lead similar lifestyles they will be subject to the same sorts of “Selection Pressures” and evolve many of the same features, so they may come to resemble each other even though not closely related at all.
SE
R
Survivors breed and pass on their characteristics. Over generations the species evolves to be using a slightly different resource. Therefore, it is no longer in competition and both species can survive... a “Win-Win” situation.
Photo by Natasha Whiteley
TIME (years)
However, this is not the only possible outcome. If among the natural variations within either species there are some individuals who are perhaps less fussy about their food or nesting sites (what ever the resource might be) and who can survive on slightly different foods, or utilize different nesting sites, then they will have an advantage to survive.
This outcome is called “Resource Partitioning” and is commonly observed in nature. For example, the many grazing species of the African plains avoid competition because each has slightly different plant preferences among the various grasses, shrubs, etc. HSC Biology Topic 2 copyright © 2005-2007
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Island 3
From one ancestral species of finch over a dozen different species evolved, one on each of the islands in the group. The key to this “Adaptive Radiation” is the fact that once a population of finches colonized a new island they were isolated from other populations. Movement of birds between islands must have been a rare event, and each population was effectively cut off from other populations.
The DDT in the chemical environment acted as a “Selecting Agent” resulting in the evolution of the insects by natural selection and survival of the fittest.
Species using resource. Population stable
Island 2
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A classic example is the shark (a fish), dolphin (a mammal) and the extinct ichthyosaurus (a reptile)
All 3 animals are (were) fast-swimming fishhunters of the ocean. All evolved the same streamlined shape, powerful tail, dorsal fin and sharp teeth to suit this lifestyle. The resemblance is superficial, and they are very different in the details of body structure and metabolism. For example, the shark is a gill-breathing ectotherm, while the dolphin is a lung-breathing endotherm. There are many other examples of such convergence. www.keepitsimplescience.com.au
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Worksheet 1
Part B The Theory of Evolution
Fill in the blanks. Check your answers at the back.
The Theory of Evolution was developed independently by a)........................................... and ................................................. It can be summarized in 5 main points: 1. All organisms produce b)........................................................ 2. Every species shows c)........................................................... 3. d)”............................................................” occurs to choose the survivors. 4. Those organisms e).................................................... to the environment survive and f)........................................................ This is called g)”.........................................................................” 5. Over generations, the species h)............................................ because the selected characteristics are being passed on in greater proportions.
Part A Evidence of Evolution The most important, direct evidence for Evolution comes from a)....................................................., the study of b)..................................... This shows that life on Earth has changed from being c)........................... complex to become d).............................. complex, and showing e)..................... variety to f)............................... variety and becoming more and more like g).......................................................... over a period of h)......................................... of years. Initially scientists could only place fossils in i)..................................... time order by j).......................................... the fossils from one place to another. The technique of k)........................................ dating has allowed absolute ages to be measured.
“Natural Selection” refers to the way that the forces of nature choose who survives. These “forces of nature” could include disease, i)............................... and ......................................., and any factors in the environment. “Survival of the Fittest” refers to how the organisms j).............................................................. to the environment will survive and k).......................................................... Variation within a population is vital so that if the environment changes, there are more likely to be some individuals who will l).................................................. and m)..................................................
An important type of fossil is a l)............................................ form, which is an “in-between” organism. A good example is m)........................................................ which lived about 100 MYA. It appears to be a n)...................................................... in the process of evolving into a o)....................................... Biogeography is the study of p)............................. ............................... It gives indirect evidence for evolution because the distribution of species can be best explained by evolution.
Changes to actual populations have been observed. The n)............................. Moth of England was observed to undergo a population change over generations due to o)..................................... pollution changing the background colours of its environment. This led to a change in the ratio of p)..................................................................................... Another example was due to a chemical change in the environment. When the insecticide q)....................... was widely used it acted as a r)”.............................................. agent”, causing mant pest species to evolve to become s).................................................................
Comparative Embryology reveals that vertebrate embryos are all q)......................................................... to each other, even though each animal grows up to be r).................................................... This is explained by evolution as being due to these animals all evolving from a s).............................................................................. Similarly, the study of Comparative Anatomy shows much the same. For example, the forelimbs of vertebrates have the same t).................................................................... even though each animal uses the limb u)......................................................................................... This “5-digit” limb structure is called v)................................................................
Competition for resources can result in one species losing and becoming t).................................. However, another possible outcome is called “Resource u).............................................” which results if one species evolves to avoid the competition by v)................................................................................................... “Divergent Evolution” is when one species w)....................................... into ........................................... This happens when different populations of a species become x).............................................. from each other. Each population is subject to different y).......................................... pressure due to the differing environments.
Biochemistry reveals that all organisms have the same genetic code in their w)......................... and use the same x).................................... to control their metabolic processes. This is evidence of y)............................................. ........................................................
“z).....................................Evolution” is when quite unrelated species evolve to aa)........................................... each other. This can happen because they live in ab).................................. environments and so natural selection favours evolution of similar ac)............................................. A good example is the shark and ad)............................................ which have many common ae)................................................ even though they are not af)................................................. to each other.
Selective Breeding of domesticated plants and animals shows that z)................................................................................ WHEN COMPLETED, WORKSHEETS BECOME SECTION SUMMARIES
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2. MENDELIAN GENETICS In Charles Darwin’s lifetime there were 2 parts of his Theory of Evolution that no-one could explain. Where Does Variation in a Species Come From? (That will be explained later in this topic) How Are Characteristics Passed On to Offspring? ... that will be explained right now!
Next, he self-pollinated the plants of the F1. SELF
TALL F1 plant
X
The Experiments of Gregor Mendel Mendel was the Abbot of a monastery in what today is the Czech Republic. He was trained as a teacher and was not a professional scientist, but became interested in discovering how inheritance works. The monks grew most of their own food, so Mendel worked his investigations into the day-today vegetable gardening by choosing to experiment with garden peas. F2 (2nd Generation) Offspring occurred in a ratio of 3:1
First he bred his pea plants over several generations to select plants that were “pure breeding” for certain contrasting characteristics, such as... TALL
v
PURPLE FLOWER
DWARF
v
Mendel didn’t do this with one or two plants, but with many. His 2nd generation groups of offspring totalled thousands of plants, not just a few.
WHITE FLOWER
He got the same result with flower colours, seed shapes, seed pod colours, and so on. In every case the first generation always took after one parent completely, and the second generation always appeared (in their thousands) in a ratio of very close to 3:1.
Each type was “pure breeding”, meaning that if they were self-pollinated they always produced offspring of exactly the same type as themselves.
Mendel’s Explanation of the Results (Using Modern Terminology) • Each characteristic is produced by “factors” (we now call them genes) carried by the plants. For example, there is a gene for tallness of stem, and a corresponding gene for dwarf stem. There is a gene for purple flower and another for white flowers, and so on for other characteristics.
Then he cross-pollinated 2 contrasting types to obtain “hybrid” (cross-breed) offspring. The result was that all the offspring showed the characteristic of one parent and none took after the other. For example: DWARF
TALL
The genes which control “opposite” forms of the same characteristic are called “alleles”, or “allelic genes”. So the genes for “tall” and “dwarf ” are alleles. Genes for “purple flower” and “white flower” are alleles.
X
“X” is short-hand for “crossed with”
• One of the alleles is “Dominant” over the other, which is said to be “Recessive”. We usually use letters of the alphabet to designate this: Tall ( T ) is dominant to Dwarf ( t ) Purple flower ( P ) is dominant to white ( p ) • Each plant carries 2 genes for a characteristic. The 2 genes may be the same as each other (“homozygous”) or different to each other (“heterozygous”).
F1 (1st Generation) Offspring were ALL TALL
Mendel explained this result by suggesting that the “factor” (GENE) which caused “Tallness” was DOMINANT to the gene for “Dwarfism”. Dwarf is RECESSIVE to Tall. HSC Biology Topic 2 copyright © 2005-2007
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Example: for the height characteristic, the possibilities are: TT = homozygous, Tall plant Tt = heterozygous, Tall plant, because T dominant tt = homozygous, Dwarf plant (cont...) 10
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Mendel’s Explanation of the Results (...continued) • Although each plant carries 2 genes for each characteristic, only one gene is passed into the gametes (pollen or ovules). Each parent passes on one gene, so the offspring gets one from each parent and gets back to having 2 genes for the characteristic.
Reasons for Mendel’s Success Gregor Mendel had discovered the basic mechanism of inheritance. Scientists before him had studied inheritance, but had failed to discover the simple patterns. Why was he successful?
Which one is passed on is completely at random.
• He chose simple, easily-identified characteristics which occurred in just 2 forms...e.g., height; either tall or dwarf
Now, we re-examine Mendel’s experiment using symbols to trace the genes through the generations:
• He began with “pure-breeding” parent plants
These are “Phenotypes”... descriptions of the outward appearance
Mendel’s “Pure-Breeding” parent plants were homozygous for each trait... 2 genes the same
PARENT PLANTS
Tall TT
These are “Genotypes” the actual genes present
T
Dwarf tt
Reasons for Mendel’s Work Being Ignored Mendel published his results in 1865 in a German natural science journal and received instant oblivion. Why?
Only 1 gene is passed into gametes
T
• He was not a recognised scientist, and had no contact with the “scientific establishment” of his time. His work was read by only a small circle of people, who failed to see its significance.
t
GAMETES
Fertilization
All the F1 offspring receive this combination of genes
Self-pollinating the F1 plants is the same as crossing with the same genotype
x
• He bred large numbers of plants so that the numbers of offspring were statistically significant. (If he’d produced just a few offspring then random chance could have produced confusing results)
• His work was written only in German, while all the “important” science of that time was appearing in English or French, in well-known journals in England, France & USA.
ALL THE F1 OFFSPRING ARE TALL, BUT HETEROZYGOUS
Tt Tall
It was not until well after Mendel’s death that in 1900 his work was “discovered” by scientists, and his great contribution was recognised. He is now known as the “Father of Genetics”.
Next, the F1 plants were selfpollinated to produce the F2 Tall Tt
t
x
Possible GAMETES
Tall Tt
T
The Punnett Square The “working out” of the cross as shown at the left can be a bit messy and confusing. A scientist called Punnett invented a simpler method which you must learn to use.
t
The “Punnett Square” working below shows the same cross... the formation of the F2 plants in Mendel’s experiment.
possible fertilizations
TT TALL
Tt TALL
Tt TALL
Ratio of Phenotypes
Genotypes of parents
tt DWARF
List of all possible gametes
3 Tall : 1 Dwarf
This explains why Mendel observed a ratio of about 3:1 in the plants of his F2 offspring.
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The phenotypes of offspring are written as a ratio or percentage
11
Tt
x
T&t
Punnett Square table shows all the possible offspring genotypes
Note that his experimental ratios were approximately 3:1, but not exactly 3:1. This is because the actual combinations of gametes at fertilization occur at random. He bred large numbers of plants and so his actual ratio was very close to theoretical. HSC Biology Topic 2 copyright © 2005-2007
Tt
T&t
gametes
T
t
T
TT
Tt
t
Tt
tt
Phenotypes of Offspring Tall : Dwarf 3:1 75% : 25%
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Monohybrid Crosses
Sample Problem 2 In drosophila fruit flies, the allele for long wings (L) is dominant to the allele for short wings ( l ). A pure breeding long winged fly was crossed with a short winged fly. Their offspring were allowed to mate among themselves to produce a second generation. There were 240 flies in the F2. Predict how many of each phenotype would occur.
“mono” = 1 (referring to just one characteristic) “hybrid” = cross-breed, when 2 “types” are crossed It has been found that there are many characteristics, in all sexually-reproducing organisms, which are inherited just the way Mendel discovered... these are cases of “Mendelian Genetics” and you need to be able to predict the outcomes of simple crosses.
Solution To work out the F1, a punnett square is not really needed.
Study the following examples to help you work through the next worksheet.
Parents genotypes
Sample Problem 1 In mice, black coat (B) is dominant to albino (b). Predict the outcome of mating a heterozygous black mouse with an albino.
Gametes possible
Solution
Then, for the F2: Bb
Parents are
x
B&b
LL x ll L only
Ll
b
B
Bb
Bb
b
bb
bb
Ll
x
L&l b&b
b
l only
All the F1 flies must be Genotype: Ll Phenotype: 100% Long winged
bb
gametes
and
L&l
gametes
L
l
L
LL
Ll
l
Ll
ll
Phenotypes of Offspring Long Wing : Short Wing 3:1 75% : 25%
Phenotypes of Offspring Black : Albino 1:1 50% : 50%
If the F2 comprises 240 flies, we should expect close to: You will soon come to realize that only 3 outcomes are possible: 100% : zero 50% : 50% 75% : 25%
180 long wing flies
and
60 short wing flies
(However, this is a statistical prediction only, and we should not be surprised if the actual numbers were (say) 190 to 50.)
NOW TRY THE PROBLEMS IN WORKSHEET 2
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Pedigrees (Family Trees)
Interpretation In Generation I, individuals 1&2 were both tongue-rollers. They had 4 children, a daughter and 3 sons. Most of the kids can roll their tongues, but son “4” cannot.
Another skill you must learn is how to read, interpret and construct a pedigree diagram. This is a diagram showing the inheritance of a trait through a family. It is used particularly with human families to trace some characteristic over a number of generations.
This means the inability to tongue roll must be recessive. (Whenever a child shows a trait different to both parents, the child’s phenotype must be recessive.)
Symbols Used in Pedigree Diagrams
Therefore, tongue-rolling ability must be dominant.
Male with trait being studied
Male without trait
Female with trait
Female without trait
We can now assign symbols...
tongue-rolling (R) non rolling ( r ) ...and work out most people’s genotypes: 1 2 3 4 5 6 7 8 9 Rr Rr ? rr ? Rr rr rr rr
Horizontal connections are “marriage lines”. Vertical lines lead to children of that couple. Each generation is numbered by Roman Numerals. Individuals may be numbered for identification
(Individuals “3” & “5” might be either “RR” or “Rr”. ... more information needed to be sure which) Questions & Answers How can we be sure that parents 1 & 2 are both “Rr” (heterozygous)?
Example In humans, some people can “roll their tongue” while others cannot. This is passed on by simple Mendelian Inheritance. Here is a pedigree of a family showing how this trait was passed on. 1
Generations
A: Since they produced son “4” who is a non-roller (and has to be genotype “rr”) both 1 and 2 must be carrying the recessive gene. Therefore, both must be “Rr”. Can we be sure that son “6” is “Rr” and NOT “RR”?
2
A: He married a non-roller (rr) and both the children in generation III are non-rollers. Therefore, son “6” must have passed on a recessive gene to his children. He must be heterozygous (Rr) to do this.
I
II
3
4
5
7
6
If “6” and “7” had another child, could it be a tongue roller? What’s the chance?
A: Yes. The cross is Rr x rr. If you work out a punnett sqare for this, you will see that the expected outcome is 50% rollers and 50% non-rollers. The chance for the next child is 50% either way, and is NOT affected by the fact they have already had 2 nonroller children.
III 8
9
TRY THE PROBLEMS IN WORKSHEET 2
Uses of Pedigree Diagrams Pedigree diagrams are used to study human inheritance because it is not moral or ethical to carry out breeding experiments on people to discover which traits are dominant/recessive. Some human disorders are inherited. Examples are haemophilia (in which blood will not clot properly) and colour-blindness (inability to distinguish certain colours). Health professionals can study affected families by compiling a pedigree chart, then advise people about the risks to future children. This allows people to make informed decisions about family planning.
Sarah can... Nathan can’t HSC Biology Topic 2 copyright © 2005-2007
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Worksheet 2 When Complete, Use as a Section Summary Part A Fill in the blanks
(continued...) c) Use a punnett square to predict the outcome of each of the following crosses. In each case, state the expected phenotypes of the offspring as a percentage. i) Gg x Gg ii) Gg x GG iii) gg x GG iv) Gg x gg
Gregor a)............................... was a monk who experimented with b)..................... plants and discovered the basics of how characteristics are c)................................................ He started with plants that were d)......................-breeding for 2 opposing traits, and crossed them. For example, he crossed pure-breeding tall-stem plants with e)......................................, f)..........................-stem plants. In the first generation, or g)............., the offspring plants were 100% h)..................................... When these were allowed to i).........................-pollinate, the F2 (which means j)................... .............................................) showed a ratio of k)....................... of tall to dwarf plants.
2. In rats, black fur (B) is dominant to brown (b). a) If a pure-breeding black rat mated with a pure-breeding brown rat, what would be the i) genotypes of the offspring? ii) phenotypes of the offspring? b) One of the offspring from the cross described in part (a) was crossed with a brown rat. i) Use a punnett square to predict the outcome. ii) In fact, they produced 7 babies; 5 black & 2 brown. Is this result surprising? Explain your answer.
He explained these results as follows: Each trait is determined by “factors” we now call l)......................... The genes which control the “opposing” traits of a characteristic are called m)................................, or n).................................. genes. For each characteristic, an organism always has o)........... genes, which may be p)................................. (homozygous), or may be different (called q).........................................) In sexual reproduction, the r).................................. (eg sperm/eggs) contain only 1 of the genes for each trait. When the gametes unite at s).........................................., one gene from each t).............................. are brought together in each offspring.
3. In humans, some people have little fingers that are straight, while others have curved little fingers. This characteristic is inherited by simple Mendelian inheritance. Study the following pedigree diagram, then answer the questions which follow.. Shaded shapes represent curved little fingers. 1
2
4
3
I
One of the alleles is dominant to the other, which is called u)........................................ The v)...................................... gene will always show its effect, but the recessive one can only be expressed in the case where it is w)......................-zygous. The appearance of an organism due to its genes is called the x)..........................................., while the “genotype” is the y).................................................................................................
II 5
6
7
8
9
III 10
11
Mendel was successful, where others had failed, because: 1. he chose z)...................................................... characteristics 2. he began with aa)............................................ parent plants 3. he bred ab)................... numbers of plants so the numbers of offspring would be ac)........................................ significant.
a) Is the curved little fingers trait dominant or recessive? Explain your answer referring to specific individuals above.
However, Mendel’s achievements were not recognised because he was not ad)............................................................ and because his findings were published in an obscure journal written in ae)........................................... and did not come to the attention of the scientific community until after his death.
c) Extra information: individuals 2 & 4 are homozygous. Using the symbols chosen, work out the genotypes of everyone in the pedigree, as far as is possible.
Part B
b) Assign the letters “S” and “s” appropriately to the 2 alleles operating in this pedigree.
d) Couple 1 & 2 had children who all have straight fingers. Was there any chance they might have had a child with curved little fingers? Explain your answer.
Mendelian Genetics Problems
e) Person 5 later married a girl with curved little fingers. Use a punnett square to predict the finger shapes of their children.
1. In pea plants, green seed pods (G) is a dominant trait over yellow seed pods (g) a) What is the phenotype of a plant, if the genotype is: i) GG? ii) gg? iii) Gg?
f) In fact, person 5 and his wife had 2 beautiful little girls both with straight fingers. Is this possible? Is your prediction wrong?
b) What is the genotype of a plant with seed pods that are: i) green, and it is pure-breeding? ii) heterozygous? iii) yellow? HSC Biology Topic 2 copyright © 2005-2007
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3. CHROMOSOMES & DNA STRUCTURE Chromosomes
Mitosis & Meiosis
Mendel’s discoveries were “re-discovered” by mainstream science in 1900. At about the same time, new techniques in using stains to highlight specific parts of the cell had led to the discovery of chromosomes. The combination of Mendelian Genetics + Chromosomes was the next big breakthrough. Things to know...
You should already be familiar with the difference between these processes of cell division in terms of their outcomes. Now look more carefully at what happens to the chromosomes during each process. ORIGINAL BODY CELL with 4 chromosomes (2 homologous pairs)
• Chromosomes are thread-like structures in the nucleus of cells. They only become visible (to a light microscope) during cell division. • Chromosomes have genes along their length. There may be 1000’s of genes on a chromosome.
This is how the genes could be located on some chromosomes.
Meiosis In BOTH processes, the chromosomes are first duplicated, to form double chromosomes
Homologous pair a A
Consider 3 sets of genes. Genotype is AaBbCc. (heterozygous for each characteristic)
In this case, 2n = 4
Mitosis
• Chromosomes occur in pairs. Chromosomes in a pair are called “homologous”. Homologous chromosomes correspond with each other by carrying allelic genes in corresponding locations. Study this diagram to get the idea.
DIPLOID” This cell is “D 2n”) (abbrev. “2 (chromosomes in pairs)
b
B
C
Chromosomes line up in their homologous pairs
Chromosomes line up in a single line
c
Another Homologous pair
Notice that homologous pairs correspond with each other, but are NOT identical.
Homologous pairs have separated, and cell divides in 2.
Chromatids separate
• Before cell division, each chromosome is duplicated.
Now cells divide again, and separate the chromatids
Study the diagram. Homologous pair of Double chromosomes
A
Each strand of a double chromosome is called a “Chromatid”. The chromatids in a double chromosome are identical... (look at the genes in the diagram) but the homologous partners are not identical... merely corresponding.
A
C
C
c
Cell Divides in 2
b
b
B
B
a
a
Haploid” means half the “H chromosome number. The chromosomes are not in pairs.
The original and its exact copy remain attached to each other, as a double chromosome.
c
DIPLOID 2n = 4
Indentical Chromatids in each chromosome
2 Daughter cells
Identical to each other and to parent cell
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4 Gamete cells
HAPLOID n=2
NOT identical to each other, nor to parent cell
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Chromosomes & Mendel’s Genes
The Structure of DNA
In 1902, two scientists independently noticed a similarity between the way that chromosomes behaved in meiosis, compared to how Mendel’s genes were inherited.
Like many biological molecules, DNA is a polymer, made of many smaller units which are joined in long chains. The basic unit of DNA is a “Nucleotide”. (named for nucleus)
Walter Sutton (USA) and Theodor Boveri (Germany) had both been studying meiosis using new staining techniques which made chromosomes more visible. Both were aware of Mendel’s discoveries, which had been “re-discovered” just 2 years before.
A NUCLEOTIDE
Chromosomes During Meiosis
Mendel’s Genes
Begin in homologous pairs
Two genes for each characteristic
Pairs separate in meiosis
The 2 genes separate in gamete formation
Gametes have only 1 of each chromosome pair (haploid)
Gametes have only 1 of each gene pair
At fertilization, each gamete supplies one chromosome, so the zygote gets back to having paired chromosomes (Diploid)
At fertilization, each gamete supplies one gene, so the offspring gets back to having two genes per characteristic
Phosphate group
“Base” molecule There are 4 different bases, so 4 nucleotides are in DNA
Sugar (Deoxyribose)
The 4 different bases are usually known just by the first letter of each name: A Adenine C Cytosine G Guanine T Thymine DNA molecules are composed of 2 strands of nucleotides (one running “upside-down” compared to the other) which are joined by the bonding between “base” molecules. Two Strands of Nucleotide Chains
The obvious similaries made it clear that the genes must be located on the chromosomes in the cell nucleus.
T
A
Chromosomes Contain DNA
Bases
Chemical analysis reveals that chromosomes are made of Deoxyribonucleic Acid (DNA for short) wrapped in proteins.
C Bases
Simulated Photograph of Human Chromosomes
G
T Part of a DNA molecule
A-T T bonded
G C-G G bonded
C
A Notice that the only base combinations are A-T T and C-G G
One Nucleotide A section of DNA carries a code to specify one feature of the organism; = a gene
Finally, the entire molecule is corkscrewed into a “double helix”, rather like a spiral staircase or ladder.
We now know that DNA is a double helix-shaped molecule which carries a chemical code... it is a gene. Each chromosome contains 1000’s of DNA molecules (wrapped in protein) each one specifying one of the organism’s many hereditary traits. HSC Biology Topic 2 copyright © 2005-2007
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Sugars & phosphates are the side rails
Bases are the “steps” of a ladder
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Non-Mendelian Inheritance
Sex-Linkage Inheritance
Gregor Mendel discovered the basics of Genetics, but it was found early in the 20th century that genes don’t always work in that simple “Mendelian” fashion.
What Thomas Morgan noticed in 1910 was that the eye colours he was studying in Drosophila fruit flies were not being inherited equally by males and females... the traits were linked to the sex of the flies... “sex-linkage”.
Thomas Morgan began experimenting with Drosophila fruit flies and quickly found they were ideal for genetics experiments. They bred quickly in large numbers, were easy to feed and keep in small bottles, and showed many different genetic traits that could be studied.
The common and normal eye colour in the flies is red. Morgan discovered a male fly with white eyes. He set out to do a “Mendel-type” breeding experiment: White-eyed Male
In 1910, in an experiment involving flies with different eye colours, Morgan realized that the way this characteristic was being inherited depended on the sex of the fly... males and females were inheriting eye colour differently.
PARENTS
Red-eyed Female X
F1 generation
What Determines Sex? In humans, and in many other species, sex is determined by a special pair of chromosomes... the “sex chromosomes”.
All have red eyes. Morgan concluded (correctly) Red is DOMINANT, White RECESSIVE
In a normal human body cell there are 46 chromosomes arranged in 23 homologous pairs. Of these, 22 pairs are called “autosomes” and are the same size and shape in males as in females.
F1 flies were allowed to mate among themselves
F2 generation
The 23rd pair are the “sex chromosomes”: Females have a matching pair of chromosomesthat are known as “X” chromosomes. A female is described as “XX”
Females 100% red-eyed. Males 50%: 50% What’s going on? Morgan produced hundreds of flies in the experiment so, like Mendel, his results were statistically significant.
Males have one “X” chromosome, and one smaller “y” chromosome. Males are described as being “Xy”
The Explanation: The genes for eye colour is carried on the X chromosome. The dominant (Red) gene can be designated as “XR ” The recessive (White) gene is “Xr ” The male “y” chromosome does NOT carry one of these alleles at all.
How these chromosomes are passed on to children can be shown using the Punnett Square diagram to track, not genes, but the sex chromosomes:
The possible female (XX) genotypes & phenotypes are: Notice that females can only pass on an X chromosome in their eggs. Male’s sperm cells can either carry an X or a y Which type of sperm fertilizes the egg determines the sex of the baby.
HSC Biology Topic 2 copyright © 2005-2007
Father
Mother
Xy
x
X&y
XRXR XRXr XrXr
XX X&X
gametes
Red eye female (homozygous) Red eye female (heterozygous) White eye female
The possible male (Xy) genotypes & phenotypes are:
X
X
X
XX
XX
Girls
y
Xy
Xy
Boys
XRy Xry
Red eye male White eye male
Note that females get 2 genes, but males only get one because their “y” chromosome lacks this allele totally. A male cannot be heterozygous for this trait and cannot have a “hidden” recessive gene.
Phenotypes of Offspring Girls : Boys 1:1 50% : 50%
WHEN DOING PUNNETT SQUARES WITH SEXLINKAGE, YOU MUST TRACK THE “X” AND “y” CHROMOSOMES... see next page. keep it simple science
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Inheritance by Co-Dominance
Morgans’s Sex-Linkage Experiment (continued...) Here is the experiment explained by Punnett Square.
Another example of “Non-Mendelian Inheritance” is known as “Co-Dominance”. This is a fairly common situation in which the 2 alleles for a characteristic do not show a Dominant-Recessive pattern, but when both genes are present (heterzygous) they are both expressed, and may result in an “in-between” phenotype.
Parents x
Xry Xr & y
The F1 are all red-e eyed. Note that females are all heterozygous.
XRXR XR & XR
gametes
XR
XR
Xr
XRXr
XRXr
y
XRy
XRy
Example: In Shorthorn cattle, there is gene (R) that causes the hair of the animal’s coat to be “red” (actually a rustybrown colour). To be red, the animal must be homozygous for this gene. Genotype RR
Phenotypes of Offspring (both sexes) Red:White 1:0 100% : 0
The other allelic gene (W) causes the coat to be white, if the animal is homozygous. Genotype WW
Then, the F1 generation were allowed to breed among themselves. Notice that the F1 females are all heterozygous red eyed ( XRXr ) and the males are all red eyed ( XRy ).
x
XRy XR & y The F2 are red : white 3:1 just like Mendel’s results, but the sexes are different.
If an animal is heterozygous, with both alleles present, neither gene dominates the other. Both genes are expressed, producing mottled patterns of red and white hair which is called “roan”.
XRXr
Genotype RW
XR & Xr
gametes
XR
Xr
XR
XRXR
XRXr
y
XRy
Xry
Note that it is best to use 2 different CAPITAL letters as symbols in this case, since neither gene is recessive. Here is what happens in a “Mendel-type” cross: PARENTS
X WW
RR
Phenotypes of Offspring FEMALES MALES Red : White Red : White 100% : 0 50% : 50%
gametes: R only F1 100% Roan
There are a number of genes in humans that are sex-linked. The best-known are 2 genetic disorders:
RW
RW
gametes: R or W
• Red-Green Colour Blindness, is where a person cannot distinguish between certain colours. • Haemophilia, is a condition in which the blood does not clot properly, putting the person at constant risk of internal bleeding. It was always a fatal condition, but in modern times people with haemophilia are treated with “clotting factors” from donated blood.
W only
RW
RW
F1 breed among themselves
F2 RR
RW
RW
WW
Both conditions are sex-linked, inherited as recessive genes on the “X” chromosome. They occur much more often in males than females.
RED : ROAN : WHITE 1 : 2 : 1 25% : 50% : 25% Try to verify for yourself the F2 result by using a Punnett Square.
TRY THE PROBLEMS IN WORKSHEET 3
TRY THE PROBLEMS IN WORKSHEET 3
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Phenotypes:
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Variation Caused by Sexual Reproduction
The Effect of Environment
The simple fact that sexual reproduction involves TWO parents, creates a lot of variation. The offspring receives genes from 2 different individuals, thereby getting a new “mix” of traits.
It’s not just an organism’s genes that produce its phenotype; the environment has an effect as well. For example, consider some of Mendel’s pea plants with different genes for stem height. Genotype TT
Variation Caused by Meiosis
IN GOOD SOIL
The process of meiosis to produce the sperm and egg cells increases variation, even before fertilization occurs.
Genotype tt
Phenotype TALL
Phenotype DWARF
Study the diagram of meiosis on page 15. Remember that homologous chromosomes are NOT identical.
Now, imagine planting them (as baby seedlings) in very poor soil, so that normal growth was not possible. Genotype TT
Each pair of homologous chromosomes line up and separate at random, and independently of all other pairs, so the number of different possible gametes is very large. In humans, with 23 pairs of chromosomes, it is possible for meiosis to produce about 8 million different combinations of chromosomes in the gametes of each person!
IN POOR SOIL
Genotype tt
Phenotype DWARF
Phenotype DWARF
The point is that the genes may control what the organism COULD grow up to be, but the environment may influence this, possibly altering the final appearance (phenotype).
But wait! ...there’s more: Crossing-Over During meiosis homologous chromosomes also swap pieces of chromatid with each other, further mixing up the possible gene combinations:
In Hydrangea plants, if cuttings are taken from a single individual (the cuttings would be genetically identical) and grown in different soils, the flowers on each cutting can be different colours. If the soil is slightly acidic the flowers will be blue, but in slightly basic soil they’ll be pink.
GAMETE FORMATION WITHOUT CROSSING-O OVER A B
b
B
b
Possible gene combinations in gametes AB
or ab
Remember, each gamete gets just one of these 4 chromatids
WITH CROSSING-O OVER a
A B
Genetics, Sex and Evolution
B
A
a b
MEIOSIS
b
Possible gene combinations in gametes AB
aB
Ab
ab
These chromosomes have exchanged pieces of chromatid with each other. This has mixed up the combinations of genes “A”, “a”, “B” and “b”.
During his lifetime, Charles Darwin freely admitted that there were 2 big gaps in his controversial Theory of Evolution... • How were characteristics inherited? When the “fittest” survive and breed, how do they pass on their “survival traits” to their offspring?
In Summary... Variation is important for Survival of the Species, and for Natural Selection to choose the survivors for Evolution.
The Science of Genetics can now explain that • Where does variation come from? Natural Selection needs differences between individuals to choose the survivors. Why is there variation anyway?
Variation in sexually-reproducing species comes from: • combining gametes from 2 different parents • Independent Segregation of all the pairs of homologous chromosomes during Meiosis • Crossing-Over process during Meiosis
Part of the answer to that is in the process of MEIOSIS and sexual reproduction
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a
a
MEIOSIS
Identical twins have inherited exactly the same genes. In the USA about 40 years ago, a famous study was done on identical twins who had been adopted into separate families and raised in different environments. The study found quite large differences between the twins in intelligence, personality, and even appearance. Presumably these differences were due to different foods, education, etc.
HSC Biology Topic 2 copyright © 2005-2007
A
In the next section, you will find out how NEW variations appear in the first place. 19
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Worksheet 3
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Baulkham Hills High School SL#802445 In 1902, two scientists t).....................................and.............................. noticed the similarities between how Mendel’s genes worked and how u)..................................... behave during v).................................... From this observation it was clear that w)......................................... ....................................................................................................................
Part A Fill in the blanks
Chromosomes are thread-like structures in the a)............................ of a cell. They are only visible during b)............................................... Chromosomes come in pairs, referred to as c).................................... The members of a c)..................................... pair are not identical, but correspond with each other because they carry d)............................................... genes in corresponding locations. Before a cell division, each chromosome is e)..................................... to make a “double chromosome”. The 2 parts are called f)............................................. and they are g)........................................ to each other.
Chromosomes have been analysed chemically and found to contain x)................................................. and ............................ It is known that the DNA molecules contain a chemical code which is a y)........................ Thus, each chromosome contains many genes, each one encoded by a molecule of z).................... The DNA molecule is a aa).......................... of repeating units called ab).................................... Each one is made of 3 smaller parts; ac)........................... and ............................... and ......................... There are 4 different “bases”, known by the intial letters of their names; ad)......, .........., .......... and .......... (letters) The DNA molecule is a double- ae)..................... shape, made of af)............... (number) strands resembling a twisted ladder. The “rungs” of the ladder are formed by 2 “bases” bonding with each other. The bases can only bond in combinations ag)...................... and ...............
In Mitosis, the chromosomes line up h)............................................... and the i)............................................... separate, so that the daughter cells are j)............................................... to each other, and to the k).................................... cell. In Meiosis, the chromosomes line up l)......................................... and the first division separates the m)............................................... pairs. Then the cells divide again to form n)................ (number) gametes, each with o)....................... the original number of chromosomes. Body cells with pairs of chromosomes are called p)........................... while gametes are said to be q)........................................... Human body cells have a total of r).................... (number) chromosomes, while egg or sperm cells have s)............... (number)
Sexual reproduction produces variations in a population because: • Offspring receive ah)........................................................................... • ai).................................................... of homol.chromos. in meiosis • the process of aj)....................................................... in meiosis.
WHEN COMPLETED, WORKSHEETS BECOME SECTION SUMMARIES Part B Problems in Sex-Linkage & Co-Dominance Check your answers at the back. 1. Refer to Morgan’s experiment with fruit flies, pages 17-18. a) Why are there 3 genotypes for female flies, but only 2 for males? b) From which parent (mother or father) does a white-eye male fly inherit his white eyes? Explain. c) i) What is the genotype of a female, who is heterozygous? ii) What is her phenotype? iii) Explain why this genotype is often referred to as a “carrier” female. d) Use a Punnett Square to predict the outcome of each cross: (You must track the X & y chromosomes. Offspring phenotypes should describe the sexes separately) i) Xry crossed with XRXr ii) XRy crossed with XrXr
3. Refer to the information about Co-Dominance, page 18. Use Punnett Squares to predict the phenotypes of calves born if: a) a roan bull mated with a red cow. b) a white bull mated with a roan cow. 4. In a particular breed of chickens, the feather colour is controlled by 2 alleles “B” and “W”. Genotype BB produces black feathers. Genotype WW produces white feathers. The heterozygous genotype BW results in black & white “speckled” feathers. Use a Punnett Square to predict the colours of chicks from: a) a speckled rooster and speckled hen. b) a black hen and a white rooster. c) a black rooster and a speckled hen. 5. Some plants have flower colours controlled as follows: There are only 2 alleles involved, but there are 3 phenotypes possible... red flowers, white flowers and pink flowers. a) Suggest how just 2 genes can result in 3 different colours. b) Suggest suitable symbols for the alleles. c) Use these symbols to write the genotype for: i) red flower ii) pink flower iii) white flower d) Use a Punnett Square to predict the phenotypes resulting from crossing a red-flowering plant with a white-flowering plant and breeding through to the F2 in a “Mendel-type” experiment.
2. In humans, a recessive gene (“Xn”) carried on the X chromosome causes colour-blindness. The “normal vision” gene can be symbolized by “XN”. Use these symbols to write the genotype of: a) a homozygous normal-vision female. b) a normal-vision male. c) a colour-blind male. d) a colour-blind female. e) a “carrier female” (heterozygous) f) A man with normal vision married a woman who is colour blind. Use a Punnett Square to predict the pattern of inheritance in their children. HSC Biology Topic 2 copyright © 2005-2007
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4. MOLECULAR GENETICS & MUTATION How DNA Structure Was Discovered
DNA Replication
By the middle of the 20th century it was suspected that DNA was probably the “genetic chemical” and it was known that it contained sugar, phosphate and the 4 bases A,C,G and T. What no-one could understand was, if DNA was the genes, how could it:
The key is the way the complementary bases bond together in the double stranded structure.
So how does the structure of the DNA molecule lend itself to replication?
• Replicate (duplicate) itself for cell division? • Control the phenotype of an organism?
sugar-phosphate “side rail” Bases
It seemed likely that the key to this problem was to find out the structure of the DNA molecule. The story of what happened is a classic example of how Science and scientists make progress using collaboration and communication.
A
G
T
C
C
A
T
C
A
G
G
T
The only combinations that will bond are A-T T
In 1953, English scientist Francis Crick had become an expert at interpreting the shapes of molecules using the new technique of “X-ray Diffraction”.
and
C-G G
This means that if you have ONE STRAND of a DNA molecule it is a “mirror-image” template for the other. If you split a DNA molecule into 2 separate strands, each strand can be used to build a new, complementary strand.
Meanwhile, at another laboratory, Maurice Wilkins (New Zealand) managed to prepare a pure crystal of DNA, and Rosalind Franklin (England) was able to get an X-Ray Diffraction image of it, but neither understood how to make any sense of the pattern it produced.
That’s exactly what happens to all the DNA in each chromosome, before a cell division occurs. DNA REPLICATION
Then a young American, James Watson, who was interested in understanding the DNA mystery, visited the WilkinsFranklin laboratory. With their collaboration, he took their data to Crick for his expert interpretation. Between them, Watson and Crick made one of the most notable scientific
GC TA
Molecule is untwisted and “unzipped” by enzymes
Base-p pairing
A
C A A T
G C C G Double-s stranded Helix
G C A T T A
breakthroughs in the history of Biology... they figured out the base-pairing, double-helix structure of DNA and realized immediately how this structure could lend itself to replication... an essential feature of a gene.
CG
C G G C
“Spare” nucleotides
CG C
AT
G
G
G T A
T
Original, doublestranded DNA
A
C
T
C T
Nucleotides match up with complementary base on original strand. Enzymes connect them in place, forming a new strand
G G
C
A
G
T AT GC CG
A
T T C
G A T
GC AT T A CG GC
When completed, each new DNA molecule is twisted back into a doublehelix shape
“old” strand
No one of these scientists could have made progress alone. Each had certain data, or skills or expertise, but only by bringing it all together was the great discovery possible. Success came from different people communicating and unselfishly sharing their knowledge and talents. HSC Biology Topic 2 copyright © 2005-2007
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“New” strand
TWO IDENTICAL DNA MOLECULES
This is how the DNA molecules, which are the genes on the chromosomes, are replicated in preparation for cell division. 21
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How Can DNA Control a Phenotype?
DNA and Protein Synthesis
To answer this question, you need to be reminded about the structure, and the role, of proteins.
The sequence of bases in the DNA molecule is a code. Each 3 bases are a “code word” (a “Codon”) which specifies an amino acid to go into the polypeptide chain.
Protein Structure has been covered briefly in previous topics. Here’s a quick revision:
DNA Strand Bases
T
A
Codon
C
T
G
G
C
A
T
C
A
A
PROTEINS are polymers of Amino Acids. Amino Acid molecules
a “Polypeptide” chain may contain 1000’s of amino acids
Amino Acids forming a Polypeptide ENZYME
Protein with precise 3-D shape
A Polypeptide is NOT a protein until it folds and twists into a final, 3-D D shape. Sometimes, several polypeptides join together to form the final protein molecule
If a polypeptide containing 1,000 amino acids is needed, then a DNA molecule made up of 3,000 nucleotide bases, will be the gene for this polypeptide. Only one strand of the double-helix of DNA is the gene. The other “mirror-image” strand is present only to allow the gene to be replicated for cell divisions. How the DNA base sequence gets to make a functioning protein which then produces a phenotype is explained by a simple model on the next page.
The exact shape of the final protein depends on the sequence of the amino acids in the polypeptide chain. There are about 20 different amino acids, and some of them are attracted to (or repelled from) each other, so how the chain twists and folds upon itself depends on exactly which ones are located where.
The Changing Definitions of a “Gene” When studying Genetics at the organism level:
a “gene” = the hereditary unit which determines one trait in the organism’s phenotype. However, at the molecular level a new definition is needed.
Protein Functions Proteins have many functions within an organism:
In the 1940’s, two American scientists, George Beadle and Edward Tatum discovered the connection of genes to proteins. Studying a genetic defect in a common fungus, they found that there were 3 different genes that could produce the same defective phenotype.
• Enzymes are all protein molecules • Structural Molecules, such as in muscle fibres, skin, hair and bone matrix are proteins.
Normal Chemical Pathway in Fungi Cells
P
• Many “Special Molecules” are proteins, such as haemoglobin (the oxygen carrier in blood) chlorophyll (absorbs light for photosynthesis) antibodies (which help fight disease) ... and many more.
Enzyme 2
R
Enzyme 3
S
Beadle and Tatum realized that each enzyme 1,2 & 3 must have a separate gene. The phenotype “S-defect” could be produced by a defect to the gene for enzyme 1, or the gene for enzyme 2, or the gene for enzyme 3. So, the new definition for a gene became: a “gene” = a unit of heredity that specifies one protein
Proteins carry out many vital functions
But now that we know about DNA, and that some proteins require more than one polypeptide chain...
Correct functioning depends on the exact sequence of amino acids in the polypeptide chain
A “Gene” is a DNA molecule which specifies one polypeptide
IT IS THE DNA WHICH CONTROLS THE AMINO ACID SEQUENCE keep it simple science
Q
P,Q,R and S are cell chemicals. If any reaction is blocked, then “S” cannot be made, and the organism has a chemical defect
In every case, it is the shape of the protein molecule which is essential to its correct functioning... Enzymes can only connect to their substrate if their shape is right.... Haemoglobin can only pick up oxygen if the shape is correct... and so on.
HSC Biology Topic 2 copyright © 2005-2007
Enzyme 1
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How DNA Causes a Phenotype:
Part 1. From DNA to Polypeptide The next step is called
The initial step is called
TRANSLATION
TRANSCRIPTION This occurs in the nucleus DNA
m-R RNA
RNA is a polymer of nucleotides, similar to DNA, but • single strand only • different sugar • one different nucleotide
gr ow in g
Each 3-b base “codon” in m-R RNA specifies one amino acid in the polypeptide chain
mfro RNA m m the ove nu s ou cle t to us ar ibo som e
P by oly ad pep di ti tio de n ch of a Am in in o
Ac id s
Ribosome moves along m-RNA
The polypeptide chain twists and folds to form a protein
Perhaps it’s a structural protein, used to build muscle or skin
Possibly it is combined with other polypeptides or special chemical groups to form...
Substrates
You should recall that the Ribosomes are organelles responsible for protein synthesis in cells
Maybe it becomes a molecule of Haemoglobin, used to carry oxygen in the blood
a functional PROTEIN molecule
It could be an ENZYME, able to catalyse a chemical reaction
Example: Eye Colour
STARTING CHEMICALS (Substrates)
Amino Acids
m-R RNA
From Polypeptide to Phenotype
The colour of your eyes is a genetically determined phenotype. The coloured pigment is made by enzymes, which are proteins, coded in your DNA
t-R RNA’s
Ribosome enzymes connect the amino acids to form a polypeptide chain
One strand of the DNA (the gene) acts as a “template” for the production of a single-stranded “messenger RNA” (m-RNA)
then, Part 2.
Amino Acids are carried into position by t-RNA (“Transfer RNA”) molecules
It could be a Hormone, which controls some aspect of Homeostasis
Enzyme
Enzyme catalyses reaction(s) to produce coloured pigment in cells of the iris of the eye
Product; a Coloured Pigment
The gene(s) for eye colour are present in every cell of your body, but are only expressed in your iris cells. Similarly, the genes for liver functions are only expressed in your liver cells, and genes to make taste buds are only expressed in your tongue. What controls which genes are expressed is still unknown in most cases. HSC Biology Topic 2 copyright © 2005-2007
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Mutation
Changing the DNA Can Produce a new Gene
In all organisms, cells divide all the time. There’s mitosis to make new cells for growth, and meiosis to make gametes for sexual reproduction. Whenever a cell is about to divide, the chromosomes are replicated and all the DNA they contain makes an exact copy of itself.
You will see at the bottom of this page that mutations can have many possible effects, ranging from absolutely nothing, to a new inheritable characteristic. How could a new characteristic happen? Imagine an accidental change to just 1 base in a DNA strand.
But sometimes mistakes happen...
Change will occur here
Original DNA Strand
• Errors can occur in duplicating the DNA • Chromosomes get broken, then incorrectly repaired and sometimes an entire chromosome just gets “lost”.
Bases
T
A
Codon
C
T
G
G
C
A
T
C
A
A
Any accidental change to the genetic material is a Mutation. Causes of Mutation Some mutations do not have any discernible cause... they are just accidents that happen. However, it is known that certain factors can increase the likelihood of mutations. These factors are called mutagens, and include:
Amino Acids forming a Polypeptide Changed base
Altered DNA Strand Bases
• Higher Temperatures. Mutation rates are higher in hotter climates than colder climates. • Certain Chemicals, for example pesticide chemicals such as the infamous “agent orange”. • Radiation, including X-rays, nuclear radiation and ultra-violet radiation from the Sun.
T
A
Codon
C
T
G
G
A
A
T
C
A
A
1 Amino Acid will be different in the polypeptide
Evidence for Mutagenic Nature of Radiation • Controlled experiments have demonstrated that genetic changes can be induced in living things (eg plant seeds, fungi, fruit flies etc) by exposing them to doses of X-rays, or nuclear radiations.
Just one amino acid being different may alter the way the polypeptide folds into its 3-D shape. This could alter the shape of an enzyme molecule, so that instead of producing (say) Blue-Eye Phenotype, it produces a new colour pigment. Blue
• Medical records from Japan following the Atom Bomb attack on Hiroshima (1945) indicate a huge increase in genetic mutations among the people in the years following.
original Enzyme
Pigment in iris
Original Phenotype (Blue eyes)
Substrate Chemical
• Countries like Australia, with a sunny climate and outdoor lifestyle, have high rates of skin cancer. These cancers are believed to be due (at least in part) to the mutagenic properties of UV light.
New Mutated Enzyme
Colour Pigment
Effects of Mutation It depends on what sort of cell the mutation occurs in.
Mutation during Meiosis Affects a Gamete
Mutation during Mitosis Affects a Body Cell
Mutation might have no effect on cell.
Cell might not be able to function properly. Cell dies. No effect on multicellular organism.
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Mutant Phenotype (new colour)
Gamete NOT involved in reproduction. No effect.
Gamete involved in fertilization Mutation might cause malfunction in cell, which becomes cancerous. Organism seriously affected, but future generations are not.
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Offspring might get new inheritable gene. New trait might be: • detrimental; less chance to survive. • an improvement; helps survival. • neither good nor bad, just different. The mutation becomes a new variation to be inherited by future generations
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Mutation and Evolution
Punctuated Equilibrium
Mutations occur at random in the DNA molecules and chromosomes. Most are detrimental to the cell in which they occur, but occasionally a mutation produces a new, inheritable trait in an offspring.
Darwin always imagined that evolution was a very slow process, taking at least 100,000 generations over perhaps millions of years, for a species to change significantly. The fact that the fossils showed significant change from one rock layer to the next was simply, in Darwin’s opinion, due to the “patchiness” of the fossil record, giving us just a glimpse here and there of the slow and steady changes going on.
Usually, new mutant genes are recessive alleles. Being recessive, they can be inherited through many generations without ever being visible in the population. Only when 2 heterozygous individuals mate and bring together a pair of recessive genes does the new phenotype actually appear.
An alternative explanation was proposed in the 1970’s. This idea, called “Punctuated Equilibrium” suggests that evolution does not occur in a slow and steady way, but in short, sharp bursts of rapid change, in between long periods of little or no change at all. It is suggested that a species might change significantly within just a few thousand years rather than millions.
The new phenotype might be quite a trivial difference... • slightly bigger ears • slightly shorter toes • a new colour pigment in the fur • a slightly longer intestine ... and so on.
EVOLUTION by PUNCTUATED EQUILIBRIUM
EVOLUTION by GRADUALISM
However, it is such small differences that give the variation to a population for Natural Selection to work on.
SLOW, STEADY CHANGES OVER TIME
but the original source of variation is MUTATION
STABLE AGAIN
Sexual Reproduction and Meiosis keep mixing the genes in new combinations,
1. All organisms produce more offspring than can survive. 2. In every species there is variation. New traits originate from genetic mutation. Traits are mixed in new combinations by meiosis and sexual reproduction, producing greater variation.
...THEN A RAPID BURST OF CHANGES NO CHANGE...
Finally, Darwin & Wallace’s Theory of Evolution is complete...
3. Natural Selection chooses the survivors. 4. The “Fittest” individuals pass on their characteristics as genes (DNA) inherited by the processes of Genetics.
Evidence for Punctuated Equilibrium • Some fossil sequences show the same species existing, apparently unchanged, for millions of years, then suddenly “disappearing” and being replaced by a different (but obviously similar) species. Perhaps it rapidly evolved into the “new” species?
5. Over generations, a species evolves. Australian animals have evolved to survive and thrive in a harsh and unpredictable environment Photo by Diana
• Studies on living populations (eg snails in grasslands) show that if the environment changes suddenly (eg because of humans clearing the land) the species can respond with a very rapid genetic shift within 20 years or so. This does not prove that a new species can evolve quickly, but lends support to the idea of rapid change. The debate continues...
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Worksheet 4 Fill in the blanks. back.
Next, the m-RNA moves out of the ai)................................... to one of the aj)............................................... These organelles are the sites of protein ak)......................................... Here the second stage, called al)......................................................., occurs. According to the code of bases on the m-RNA, amino acids are placed oneby-one in sequence to build the am)........................................................... chain. Another form of RNA, called an)................................ carries each amino acid into place. The enzymes of the ao).................................. then catalyse the reaction to join the amino acids to each other.
Check your answers at the
The discovery of the structure of DNA shows how good a)............................................ and b)............................................. contribute to scientific progress. Maurice c).................................... was able to prepare a crystal of pure DNA, and Rosalind d)............................................. was able to get an X-ray Diffraction image of the crystal, but neither of them could interpret it. James e)................................ collaborated with them, and took their data to Francis f)...................................... who had become an expert in this new technology.. Between them, Watson & Crick figured out the double-helix shape and the g)..................-pairing structure of DNA.
To cause a phenotype to occur, the polypeptide chain must then twist and fold to form a functioning ap).......................................... As an example, it could become an enzyme, which aq)............................................ a chemical reaction in the iris of the eye. The reaction might result in the production of a coloured ar).............................................. which produces the phenotype of eye colour.
The h).......................-stranded structure, and the complementary base-pairing allows DNA molecules to be i)........................................ in preparation for a cell j)..................................... First, the DNA strands k)................................................ Then each strand can act as a l)................................................ for the making of a new complementary strand.. Each base specifies what must go on the new strand, because only bases m).......... & ................ and bases n).............. & ..................... can go together. Once a new strand is built onto each “old” strand, the result is 2 o).................................... DNA molecules.
Any accidental as)................................... to a gene or chromosome is a at)............................................. Some do not have any discernible cause, but there are certain factors, called au)...................................... which increase the likelihood of a mutation. Higher av)...................................... is one, and certain aw).................................... are known to be mutagens. Various forms of radiation are mutagenic, including ax).................................. and ay).......................... and az) ................................................
Proteins are polymers of p)...................................................... A chain of p).................................................. is called a “q).......................................................... chain”. To become a functioning protein, it must twist and fold into a precise r)............................................ Exactly how the chain twists and folds depends on the exact s).............................. of the amino acids. There are about t)................. (number) different amino acids and they may attract or u)................................... each other, causing the chain to twist and fold upon itself..
Altering just one base on a ba)....................... molecule can create a new gene. One different base could cause one bb).............................-acid to be different in the bc)............................................................ chain. This in turn could alter the bd)......................................... of a protein. If this protein is an be)........................................., then the way it catalyses a reaction could change, resulting in a different chemical product. For example, this could result in a different bf)....................................... in the iris cells of the eye, and so a new phenotype for eye colour.
Some different types of proteins include v)............................... which catalyse metabolic reactions, and w)................................ proteins in muscles, skin and hair cells. In each case, it is the x)........................... of the protein which is vital to its correct functioning. This shape is due to the sequence of y)....................... acids, and these are specified by the base sequence of the z).....................................
Mutations are significant for the Theory of bg)................................ because they explain where bh)............................... in a population comes from originally. It has usually been accepted that evolution occurs with a slow and steady accumulation of changes. In the 1970’s a new idea, called bi).................................... .................................................... was put forward. This suggests that species bj)............................................... for long periods of time, and then undergo bk).................................. ..................................... of evolution.
In DNA each aa).............. (number) bases form a code word (called a ab)...................................) to specify one amino acid. The first step in the process is called ac)........................................... and involves the production of a molecule of ad)............................ To do this, the “gene strand” of the DNA is used as a ae).......................................... to build the RNA from nucleotides. Compared to DNA, the RNA is only af).............................-stranded, and has a different ag)........................................... in the “backbone” of the molecule, and one different ah).............................................. HSC Biology Topic 2 copyright © 2005-2007
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5. REPRODUCTIVE TECHNOLOGIES & GENETIC DIVERSITY Reproductive Technologies Humans have developed a number of techniques for controlling or enhancing the reproduction of crop plants, domesticated animals, and even themselves.
Cloning A “clone” is a group of organisms which are all genetically identical. The simplest form of cloning is asexual reproduction, which has been done artificially with plants for thousands of years.
Artificial Pollination This is the oldest and simplest reproductive technique. Gregor Mendel used this to control the reproduction of his pea plants, and it had probably been used for centuries before that.
Every time a plant is grown from a cutting, or by grafting, a clone is being created. Farmers and gardeners have been doing this for thousands of years. More recently, the process of “Tissue Culture” has allowed plant cloning on a massive scale. This involves taking thousands of small pieces of tissue (each perhaps just a few cells) from a “parent” plant and culturing them in a nutrient liquid in a test tube in the laboratory. Eventually they grow large enough to be planted out into soil to grow to be “adult” plants. This allows a clone of many thousands to be grown from just one parent plant.
Very simply, it involves taking pollen from one (selected) plant and dusting it onto the flower of the other (selected) parent. This gives complete control over which plants become the parents of the next generation. In modern agriculture, almost all of our cereals, fruits and vegetables are now the result of artificial pollination. New “types” are created by artificial pollination using different varieties or even different species. Such “cross-breeds” are called “Hybrids”. For example, the “nectarine” is a hybrid formed by crossing a peach with a plum.
Cloning of animals is much more difficult and the first mammal clone was not achieved until the 1990’s with the famous “Dolly” the sheep.
Once commercially important varieties are created, huge numbers of seeds are produced, again by artificial pollination of selected parent plants. This results in millions of almost identical plants being grown in crop “monocultures”.
HOW TO CLONE A MAMMAL EGG DONOR
PARENT of CLONE
Commercially it has many advantages, such as great consistency in growth rates and food quality. Later, we will discuss possible disadvantages. Body Cell
Artificial Insemination (A.I.) This is basically the animal equivalent. Semen can be collected from a champion bull, divided up into many small samples and frozen. Distributed anywhere in the world, it can be thawed out and inserted into a fertile cow with a high chance of successful fertilization.
Diploid Nucleus Extracted
Surrogate Mother
This works too, for racehorses, dogs, sheep and so on. There are huge advantages, such as entire herds of highquality beef cattle, but there may be disadvantages too... these will be discussed later.
“Fertilized” egg placed in uterus
Normal pregnancy and birth
Humans use A.I. for their own reproduction too. If a man is sterile, but he and his wife wish to have children, she can be inseminated with semen from an anonymous donor. There are “sperm banks” of frozen semen for this use. No-one has seriously suggested using this to “improve” the human race... this would be an ethical & moral issue. keep it simple science
Nucleus removed
Diploid Nucleus from body cell placed into Egg Cell
Thus a top-quality bull can become the father of many thousands of calves, thereby improving the quality of cattle herds all over the world.
HSC Biology Topic 2 copyright © 2005-2007
Unfertilized Egg
The baby lamb is genetically identical to the single parent at top left. It is an “identical twin”, except younger
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Reproductive Technologies & Genetic Diversity
How Genes are Transferred The exact mechanism of transfer depends on the species.
Artificial Pollination, A.I. and cloning all have the potential to diminish the Genetic Diversity of the species involved. All these processes, if used on a large scale, result in many offspring being produced from just a few, or even just one, parent. This means that all these offspring are closely related to each other and have less genetic diversity.
Firstly, the gene to be transferred from the “source species” needs to be isolated. This is often achieved by collecting DNA from cells of this species and chopping it into fragments using “restriction enzymes” from bacteria. These enzymes have the ability to cut DNA into sections. It often takes a lot of trial-and-error to then locate a fragment of the chopped-up DNA containing the desired gene.
Already, many of our food crops, such as wheat and rice, are “monocultures” of millions of individual plants who are “siblings” from relatively few parents. In a banana plantation, the entire population may be a clone of genetically identical plants grown from cuttings and “suckers” (asexual runners) from a single parent.
The desired fragment can then be incorporated into the DNA of the “target species” by several possible methods. • In some cases the DNA fragment can be injected into an egg of the target species. The DNA becomes incorporated into a chromosome and is an inherited gene from that point on.
Among farm animals such as beef cattle, the widespread use of A.I. to improve herds has also lessened the genetic diversity of those herds.
• Some viruses quite naturally transfer pieces of DNA from one cell to another. These can be “enlisted” as vectors to carry DNA to the target cells. The DNA fragment is inserted into a harmless virus. Then when the virus is allowed to infect a “target cell”, the DNA fragment becomes incorporated into its chromosomes.
There are many benefits to this, but there may be serious consequences too. Remember that genetic diversity (“variation”) is essential for the long-term survival and evolution of a species. If a species has little variation, then any change in the environment (eg a new disease, climate change) might adversely affect ALL the population, and leave no survivors.
• In the case of transferring human genes into bacteria, the same “restriction enzyme” is used to chop-up both the human DNA, and the bacterial “Plasmid”... a bacterial chromosome. Since both were cut by the same enzyme, the fragments match-up, and can readily join together when mixed in the presence of suitable enzymes. This technique has been in use for about 40 years, notably to produce pure human insulin for treating diabetics.
Transgenic Species As well as reproductive technologies, humans are altering the “gene pools” of species in another way... Genetic Engineering technology is able to transfer one or more genes from one species to another to form “Transgenic Species”. Some examples: Gene Transfer Human gene for Insulin hormone inserted into bacteria
Ethical Issues Concerning Transgenic Species
Purpose / Benefits Bacteria produce insulin (to treat diabetic patients) cheaply and in great quantity
While Reproductive Technologies may cause some scientific concerns about loss of genetic diversity, the main concerns about Transgenic species tend to be ethical. You need to be aware of the issues and prepared to form your own, informed opinion... there is no single correct answer.
Human genes for blood- Sheep can be milked and blood clotting factors inserted factors extracted to treat into sheep, which produce patients suffering Haemophilia blood factors in their milk Strawberry plants have received a gene from a fish, which allows them to resist freezing better
Allows strawberries to be grown in colder areas not previously possible
Crop plants have received a gene from the Pyrethrum plant which causes the crops to produce a natural insecticide chemical
Crops do not need to be sprayed with insecticides to control insect pests... any insect which eats the crop will die
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Weigh up the benefits (some listed in table at left) against the concerns some people raise... • transferring genes, especially human genes, into other species is unnatural, and wrong religiously and morally. • foods from transgenic species may have unforseen consequences for human health, such as inceases in cancer. No-one has the right to expose us all to unknown risks. • under current law, the companies who develop transgenic species own patents on them, and are making huge profits by forcing farmers to use their products or sell-out. This is seen as unethical, putting profit before people. 28
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CONCEPT DIAGRAM (“Mind Map”) OF TOPIC Some students find that memorizing the OUTLINE of a topic helps them learn and remember the concepts and important facts. Practise on this blank version.
THE BLUEPRINT OF LIFE
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Practice Questions
7. An example of “microevolution” that has actually been observed to happen, is: A. the extinction of the dodo. B. divergence of “Darwin’s finches” on the Galapagos Islands. C. convergence of the shark and dolphin. D. change in the population of the “Pepper Moth”.
These are not intended to be "HSC style" questions, but to challenge your basic knowledge and understanding of the topic, and remind you of what you NEED to know at the K.I.S.S. principle level. When you have confidently mastered this level, it is strongly recommended you work on questions from past exam papers.
Part A
8. When Gregor Mendel crossed pure-breeding tall pea plants with pure-breeding dwarf plants, and bred them through 2 generations, he found that the F2 phenotypes were: A. 100% tall plants B. 75% tall: 25% dwarf, approximately. C. 50% tall : 50% dwarf, approx. D. ratio of 3:1, dwarf : tall, approx.
Multiple Choice
1. The fossil record shows a pattern, from ancient times to recent times, of: A. more complexity to less complexity. B. greater diversity to less diversity. C. increasing complexity and diversity. D. no distinct pattern of change.
9. The genotype of a pea plant described as “heterozygous tall” would be: A. Tt B. T C. TT D. tt
2. The famous fossil “archaeopteryx” is best described as a: A. common fossil, found in many rock layers. B. “transitional form”, showing a stage of evolution. C. “index fossil”, useful for correlating between locations. D. “living fossil”; an extremely primitive bird alive today..
10. In mice, Black coat (B) is dominant to white (b). If a heterozygous mouse mated with a white mouse, you would expect A. approximately 75% of the babies to be black. B. all the babies to be black. C. approximately 50% of the babies to be white D. all the babies to be white.
3. Two types of evidence which suggest descent from a common ancestor are: A. embryos of vertebrates and the pentadactyl limb. B. convergent evolution and comparative biochemistry. C. DNA structure and the biogeography of grazers. D. natural selection and survival of the fittest.
11. The following pedigree shows the inheritance of “tooth gap” (a gap between the 2 top-front teeth) in a human family. Shaded shapes show people with a tooth gap.
4. “Natural Selection” refers to A. those organisms best suited to the environment survive and reproduce. B. factors of the environment determining which organisms survive and which do not. C. the changes to living things over many generations. D. the way that not all organisms can survive because too many offspring are produced.
1
II 5
6
7
8
9
III 10
11
It would be true to say that: A. “tooth gap” is dominant because it’s more common. B. couple 3 & 4 are both heterozygous for this trait. C. the exact genotypes of 10 & 11 are uncertain. D. individual 9 must be homozygous.
6. A species has a better chance of long-term survival during changes to the environment if: A. it reproduces asexually. B. most individuals are similarly adapted. C. the species has a lot of variation and genetic diversity . D. there are many large, strong individuals in the group. keep it simple science
4
3
I
5. The really important part of “Survival of the Fittest” is: A. being very well adapted to the environment. B. living a long and healthy life. C. being “chosen” by the environment to survive. D. reproducing many offspring.
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20. The factor least likely to be mutagenic is: A. nuclear radiation. B. herbicide chemicals like agent orange. C. ultra-violet light. D. low temperatures.
12. A pair of homologous chromosomes: A. are identical to each other. B. carry totally different genes C. will line up as a pair during mitosis. D. carry alleles in corresponding locations. 13. Meiosis produces: A. 4 haploid gamete cells. B. 2 diploid body cells. C. 4 identical gametes. D. 4 non-identical body cells.
21. The base sequence on a part of a DNA chain is ...ATTCGAGGCTAC... Which of the following statements is correct? A. This section could code for 4 amino acids. B. The complementary strand would read TAACGACCGTAC. C. This section contains 6 codons. D. The corresponding m-RNA section would be ATTCGAGGCTAC.
14. A “nucleotide” contains: A. sugar, phosphate and one of 4 possible “bases”. B. a “base”, phosphate, and one of 4 possible sugars C. amino acids in a polypeptide chain. D. base pairs A-T and C-G.
22. When genes are transferred from one species to another: A. this reduces the genetic diversity of the species. B. this is a mutation. C. this produces a transgenic species. D. the result would be a clone of the original.
15. In human sperm cells you would expect to find: A. 46 chromosomes including an Xy pair. B. 46 chromosomes including either an X or a y. C. 23 chromosomes, including an X. D. 23 chromosomes, including either an X or a y. 16. If a recessive gene is “sex-linked” you would expect: A. males and females to show the phenotype equally. B. a heterozygous female will show the recessive phenotype. C. males may inherit the trait only from their mother. D. males will pass the recessive gene on to their sons.
Part B
17. In a species of small African rodents it was noticed that 2 grey coloured animals produced a litter of babies that were about 25% white, 25% black and about 50% grey. It seems likely that: A. grey is dominant to both black and white. B. co-dominant alleles are controlling coat colour. C. a mutation occurred to produce new colours in the babies. D. grey is a recessive gene, black and white are codominant.
23. (4 marks) Explain the difference between “relative dating” of fossils and “absolute dating”, by outlining how each is achieved and what each can tell you about fossils. 24. (3 marks) Comparative Anatomy can give evidence supporting the idea of evolution. a) Outline an example of comparative anatomy that gives such evidence. b) State how this example is evidence of evolution.
18. Which of the following is NOT a source of increased genetic diversity in a species? A. Mutation in a gamete before fertilization. B. Crossing-over process during mitosis. C. Independent segregation of homologous pairs in meiosis. D. Combining gametes from different parents at fertilization.
25. ( 3marks) Briefly discuss, using an example, how advances in technology have changed scientific thinking about evolutionary relationships. 26. (5 marks) Outline the Darwin-Wallace Theory of Evolution. 27. (4 marks) Differentiate between “convergent” and “divergent” evolution, giving examples of each.
19. DNA replication is possible because: A. DNA acts as a template to make m-RNA. B. Each DNA strand makes an identical copy of itself. C. Each DNA strand makes a complementary strand. D. The polypeptide chains in DNA are “mirror images”. HSC Biology Topic 2 copyright © 2005-2007
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Longer Response Questions
Mark values given are suggestions only, and are to give you an idea of how detailed an answer is appropriate.
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33. (5 marks) In Drosophila fruit flies, the normal eye colour is red. A recessive, sex-linked gene causes white eyes. Using symbols XR, Xr and y, predict the phenotypes (separately for each sex) of the cross between a white-eye male and a heterozygous red-eye female. Show your working.
28. (6 marks) Explain the difference between each pair of words: a) Dominant & Recessive genes. b) Homozygous & heterozygous. c) Genotype & Phenotype. 29. (5 marks) Outline the reasons why: a) Mendel’s experiments were successful in discovering the fundamentals of how inheritance works.
34. (5 marks) A certain species of African rodent was described in Multiple Choice Question 17. Using appropriate (declared) symbols, predict the outcome of a mating between a grey and black animal of this species.
b) Mendel’s discoveries were ignored by other scientists for many years. 30. (5 marks) In pea plants, green seed pods(G) is dominant to yellow pods (g). A pure breeding green pod plant was crossed with a purebreeding yellow pod plant. a) Explain why all the offspring will have green pods.
35. (3 marks) a) Explain, with the help of simple diagrams if you wish, the process of “crossing over” with chromosomes. b) Explain how this process contributes to the genetic diversity of a species.
b) One of these offspring plants was later crossed with a yellow pod plant. Showing all working, predict the phenotypes (as ratio or percentage) of the offspring from this second cross.
c) State one other process which increases genetic diversity in a species. 36. (6 marks) Use a simple flow chart diagram to outline the process by which DNA controls the production of a polypeptide.
31. (8 marks) a) Use the following information to construct a pedigree diagram.
37. (5 marks) a) What is a “mutagen”?
Inheritance of red-hair in the Meggs family. Fred and Mary both have blonde-brown hair. They have 4 children, a daughter and 3 sons. Their daughter and one son are red-heads, the other 2 sons are similar to their parents. The red-headed son married a red-headed girl and they have a son and a daughter.
b) Describe 2 pieces of evidence that radiation is mutagenic. c) Describe using a simple flow chart, how a change in a DNA sequence could result in a change in cell activity.
b) State whether “red-headedness” is a dominant or recessive trait, giving reason(s).
38. (4 marks) Compare and contrast the idea of “Gradualism” in Evolution, with the theory of “Punctuated Equilibrium”.
c) Predict the hair colour of Fred & Mary’s grandchildren. Explain your answer.
39. (5 marks) a) Outline the process that can be used to make a clone of a mammal.
32. (6 marks) a) Sketch one pair of homologous chromosomes, just before a cell was to undergo a cell division. If the organism is heterozygous for allelic genes “Q” and “q”, show on your diagram the positions of all copies of these genes.
b) Explain the effect that cloning could have on the genetic diversity of the cloned species. 40. (5 marks) a) Give an example of the use of a “transgenic” species, stating: i) precisely which species are involved, and ii) the benefit or purpose achieved.
b) Draw simple sketches to show: i) how these 2 chromosomes would line up for mitosis. ii) the chromosomes in ONE of the cells resulting from mitosis. iii) how these 2 chromosomes would line up for meiosis. ii) the chromosomes in ONE of the cells resulting from meiosis. HSC Biology Topic 2 copyright © 2005-2007
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b) State an ethical concern that some people might have with the use of this transgenic species. 32
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Worksheet 2
Answer Section
Part A a) Mendel c) inherited e) pure-breeding g) F1 i) self k) 3:1 m) alleles o) 2 q) heterozygous s) fertilization u) recessive w) homozygous y) actual genes present. aa) pure-breeding ac) statistically ae) German
Worksheet 1 Part A a) Palaeontology c) less e) less g) present-day organisms i) relative k) Radiometric m) Archaeopteryx o) bird q) very similar s) common ancestor u) for different purposes w) DNA y) common ancestry
b) fossils d) more f) more h) millions (billions) j) correlating l) transitional n) dinosaur p) how living things are distributed r) very different t) bone structure v) Pentadactyl limb x) enzymes z) species can be changed
Part B a) Charles Darwin & Alfred Wallace b) more offspring than can possibly survive c) variation (genetic diversity) d) Natural Selection e) best suited f) reproduce / breed g) Survival of the Fittest h) evolves / changes i) predators, food, supply j) who are best suited k) reproduce l) survive m) reproduce n) Pepper o) industrial p) black to light moths q) DDT r) selection s) resistant t) extinct u) Partitioning v) using slightly different resources w) evolves into several / more than one x) isolated y) selection z) Convergent aa) resemble ab) similar / same ac) characteristics / adaptations ad) dolphin ae) features / adaptations af) closely related
Part B 1. a) i) green b) i) GG c) i)
b) pea d) pure f) dwarf h) tall j) 2nd generation l) genes n) allelic p) the same r) gametes t) parent v) dominant x) phenotype z) simple, easily identified ab) large ad) a recognised scientist
ii) yellow ii) Gg
Gg
iii) green iii) gg
Gg
x
G&g
G&g
gametes
G
g
G
GG
Gg
g
Gg
gg
Phenotypes of Offspring Green : Yellow 3:1 75% : 25%
Answers only for the rest of these. ii) Gg x GG: 100% green iii) gg x GG 100% green iv) Gg x gg 50% green : 50% yellow 2. a) i) BB x bb: all offspring will have genotype Bb ii) Phenotype: all will be black b) i) Bb bb x ii)Not surprizing. In such small samples, random chance can give results not exactly in the expected ratio.
B&b
b&b
gametes
b
b
B
Bb
Bb
b
bb
bb
Phenotypes of Offspring Black : Albino 50% : 50% HSC Biology Topic 2 copyright © 2005-2007
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Worksheet 3 Part B continued
3. a) Recessive. Couple 7 & 8 have straight fingers, but had a child (11) with curved little fingers. This can only happen if both parents are carrying a “hidden” gene... therefore it must be recessive. b) S = Straight, s = curved. c) 1=ss, 2=SS, 3=ss, 4=SS, 5=Ss, 6=Ss, 7=Ss, 8=Ss, 9=Ss, 10=SS or Ss(uncertain), 11=ss d) No chance of curved-finger children, because all children must receive a “S” gene from mother who is “SS”. e) Ss x ss would give 50% curved, 50% straight. f) It is quite possible. Prediction is not wrong. In small samples, random chance can result in offspring ratios that are not in agreement with the prediction. (In a large sample of offspring there should be approximately 50-50)
1. d) ii) Females 100% Red-eyed. Males 100% white eyed. 2. a) XNXN d) XnXn f)
b) XNy e) XNXn
XN & y
a) nucleus b) cell division c) homologous d) allelic e) replicated / duplicated f) chromatids g) identical h) in single file i) chromatids j) identical k) parent l) in pairs m) homologous n) 4 o) half p) diploid q) haploid r) 46 s) 23 t) Sutton & Boveri u) chromosomes v) meiosis w) the genes must be located on the chromosomes x) DNA and protein y) gene z) DNA aa) polymer ab) nucleotides ac) sugar, phosphate & base ad) A,C,G & T ae) helix af) 2 ag) A-T and C-G ah) genes from 2 different parents ai) Independent Segregation aj) Crossing-over
Xr & y
3. a)
Xn
XN
XNXn
XNXn
y
Xny
Xny
Xr
Xr
XRXr
XrXr
y
XRy
Xry
RR
x
R&R
gametes
R
R
R
RR
RR
W
RW
RW
Phenotypes of Offspring Red : Roan 1:1 50% : 50%
b) WW x RW. Phenotypes of offspring 50%White, 50% Roan. 4. a)
BW
x
B&W
XR & Xr
XR
RW R&W
XRXr
gametes
Xn & Xn
gametes
Phenotypes of Offspring FEMALES MALES 100% normal 100% colour blind vision
Part B 1. a) Males cannot be heterozygous because they only have one X chromosome. The y chromosome doesn’t carry an allele. b) From his mother, who gives him his X chromosome. Father gives y, which doesn’t carry an allele. c) i) XRXr ii) Red-eyed iii) She carries the recessive gene and passes it to her children, but does not show the phenotype of it herself. x
XnXn
Xn
Part A
Xry
x
XNy
Worksheet 3
d) i)
c) Xny
BW B&W
gametes
B
W
B
BB
BW
W
BW
WW
Phenotypes of Offspring Black : Speckled : White 1 : 2 : 1 25% : 50% : 25%
b) BB x WW gives 100% Speckled offspring. c) BB x BW gives 50% Black and 50% Speckled.
Phenotypes of Offspring FEMALES MALES Red:White Red : White 50% : 50% 50% : 50% HSC Biology Topic 2 copyright © 2005-2007
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Practice Questions
5. a) If the 2 alleles are Codominant, then there can be 3 phenotypes. b) Best to use 2 different CAPITAL letters, R & W. c) i) RR ii) RW iii) WW d) Parents: RR x WW Gametes: R only W only F1:
1. C 2. B 3. A 4. B
RW
x
R&W R
W
R
RR
RW
W
RW
WW
Worksheet 4
HSC Biology Topic 2 copyright © 2005-2007
b) collaboration d) Franklin f) Crick h) double j) division l) template n) C & G p) amino acids r) 3-D shape t) 20 v) enzymes x) shape z) DNA ab) codon ad) m-RNA af) single ah) base aj) mitochondria al) Translation an) t-RNA ap) protein ar) pigment at) mutation av) temperature ax) X-rays az) Ultra-violet U.V. bb) amino bd) shape bf) pigment bh) variation / genetic diversity bj) remain unchanged
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13. A 14. A 15. D 16. C
17. B 18. B 19. C 20. D
21. A 22. C
24. a) An example is the “Pentadactyl Limb” structure in vertebrate animals. The bone structures in the fore-limb of a whale, bat, dog, human, lizard, etc. are all essentially the same, even though each looks very different on the outside, and each is used for very different purposes... swimming, flying, walking, etc. b) This is evidence of descent from a common ancestor which had that bone structure.
Phenotypes of Offspring Red : Pink : White 1 1 : 2 : 25% : 50% : 25%
a) communication c) Wilkins e) Watson g) base i) replicated k) unzip / untwist m) A & T o) identical q) polypeptide s) sequence u) repel w) structural y) amino aa) 3 ac) Transcription ae) template ag) sugar ai) nucleus ak) synthesis am) polypeptide ao) mitochondria aq) catalyses as) change au) mutagens aw) chemicals ay) nuclear radiation ba) DNA bc) polypeptide be) enzyme bg) Evolution bi) Punctuated Equilibrium bk) rapid bursts
9. A 10. C 11. B 12. D
23. Relative Dating is done by correlating fossils from one place with those elsewhere. It allows scientists to put things in order, from oldest to youngest, but the actual ages cannot be determined. Absolute Dating involves measuring the radiation from naturally occurring radio-isotopes. These “decay” at known rates, so the amount of radiation remaining in a rock or fossil allows the actual age in years to be determined.
R&W
gametes
5. D 6. C 7. D 8. B
Part B Longer Response In some cases there may be more than one correct answer possible. The following “model” answers are correct, but not necessarily perfect.
100% RW (Pink) RW
F2
Part A Multiple Choice
25. Sequencing the DNA of different species allows their degree of “relatedness” to be determined very precisely, since closely related species have very similar DNA. This has revealed some surprizes that have changed the thinking on evolutionary relationships. For example, it has previously been thought that lizards and crocodiles (both classified as reptiles) are more closely related to each other than to birds. DNA analysis reveals that birds are more closely related to crocodiles than either is to the lizards. 26. (Best in 5 numbered points) 1. All species produce more offspring than can survive. 2. There is variation within each species... not all the same. 3. “Natural Selection”: the factors of the environment choose which individuals survive. 4. “Survival of the Fittest”: those best suited to the environment survive and reproduce. 5. The “survival characteristics” get passed on to offspring so each generation is slightly different than before. Over many generations the species changes... it evolves. 27. Divergent evolution is when one ancestral species evolves into 2 or more different species. An example is the finches of the Galapagos islands, where a different species of finch has evolved on each separate island, all from a single ancestral species. Convergent evolution is when different species evolve to resemble each other, even though they are not closely related. An example is the shark & dolphin which are both streamlined, fast-swimming look-alikes, but are not related in an evolutionary sense. 28. a) Dominant gene will always be expressed. Recessive gene can only be expressed when no other allele is present (i.e. if homozygous for the recessive gene)
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b) Homozygous means having 2 copies of the same allele for a particular trait. eg, AA, or bb. Heterozygous means to have 2 different alleles for the trait. eg, Aa c) Genotype is the description (usually in symbols) of the actual genes present for a trait. eg “Aa”. Phenotype is the outward appearance caused by the genes for that trait. eg “Purple flowers” or “Dwarf stem”.
Homologous pair of Double chromosomes
Q
29. a) Mendel’s experiments were successful because: • he chose to study simple traits that occurred in just 2 alternative forms, such as Tall v. Dwarf. • he began with pure-breeding parent plants. • he bred large numbers of offspring so the results could be interpreted statistically. b) His work was ignored because: • he was not a recognised scientist, but a monk in an obscure monastery. • his findings were published in a little-known journal, in German, so were not read by many people (and those that did, didn’t realize the significance).
b)
q
Q
i) in a single line
q
ii)
iii) in pairs iv)
30. a) Each of the offspring will receive a gene from each parent and so all will have genotype “Gg”. Since G is dominant, it will be expressed (= green pods), while the recessive “g” gene will have no effect. b) Gg G&g
Xr & y
g&g
gametes
g
g
G
Gg
Gg
g
gg
gg
Fred
XR
Xr
Xr
XRXr
XrXr
y
XRy
Xry
GREY
Mary
BLACK
BW
I
x
B&W 3
4
5
7
6
III 8
keep it simple science
BB B&B
gametes
B
B
B
BB
BB
W
BW
BW
9 Phenotypes of Offspring Black : Grey 1 : 1 50% : 50%
b) Red-headedness must be recessive, because Fred & Mary are not red-heads, but had red-headed children. They must both carry a recessive gene. c) Must both be red-heads, since their parents are. Since it’s recessive, both 6 & 7 must be homozygous for red-hair and must pass on genes to children 8 & 9, who also must be homozygous. HSC Biology Topic 2 copyright © 2005-2007
XR & Xr
gametes
34. Gene symbols: B= Black, W=White
31. a)Shaded shapes are red-heads
II
XRXr
Phenotypes of Offspring FEMALES MALES Red : White Red : White 50% : 50% 50% : 50%
Phenotypes of Offspring Green pod : Yellow pod 1:1 50% : 50%
Generations
x
Xry
33.
gg
x
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Baulkham Hills High School SL#802445 38. Similarity: both are models describing evolution of a species. Difference: “Gradualism” = evolution occurring slowly and steadily over relatively long time periods. “Punctuated Equilibrium” = species remains unchanged for long periods, then evolves in a rapid burst.
35 a) “Crossing-over” is when homologous chromosomes exchange corresponding parts of a chromatid with each other. b) This mixes up the combinations of genes on each chromatid, and results in more variety of gene combinations in gametes, and in offspring. c) Mutation / Independent Segregation of homologous chromosomes in meiosis / Receiving genes from 2 different parents during sexual reproduction. 36.
TRANSCRIPTION
DNA
39. a) • Body cell taken from “parent”, and nucleus extracted. • Egg cell obtained and nucleus replaced with nucleus from “parent” cell. • Egg cell implanted in uterus of surrogate mother, where normal pregnancy and birth occur. Baby is clone of “parent” individual. b) If used on a large scale, cloning would reduce genetic diversity because less parents are involved and more offspring would be identical.
m-R RNA Moves to a Ribosome
TRANSLATION
Polypeptide
m-R RNA controls sequence of amino acids joined together
40. a) i) Gene from a fish transferred into strawberry plants. ii) Makes strawberries frost-resistant, allowing them to be grown in areas not previously possible. b) Some people may be worried about long term health effects of eating transgenic foods. (In this particular case, vegetarians might see it as unethical that the strawberries contain an animal gene.)
37. a) Mutagen = something that CAUSES a mutation. b) 1. Controlled experiments have demonstrated that mutations are caused in plant seeds, insects, etc which are exposed to nuclear radiation or x-rays. 2. There was a large increase in human mutations and genetic abnormalities in Hiroshima after the atomic bomb attack of 1945, and the Chernobyl accident in Ukraine 1986. c) Altered Altered Altered m-R RNA DNA Polypeptide
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Altered Enzyme
Change in cell’s activity
causes different chemical product to form
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