Biology, Chapter Twelve, Notes

UNIT THREE: GENETICS AND BIOTECHNOLOGY (Text from Modern Biology, Holt, Rinehart, and Winston)

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Chapter Twelve (Inheritance Patterns and Human Genetics) SECTION ONE: CHROMOSOMES AND INHERITANCE CHROMOSOMES Francis Collins and his lab group discovered the gene responsible for cystic fibrosis (CF), which is an often fatal genetic disorder in which thick mucus builds up and blocks ducts, making it difficult to breath. Due to work by geneticists in the early 1900s, they were able to study the CF gene. Early Work Researcher Thomas Morgan began experimenting with the small fruit fly Drosophila melanogaster in the early 1900s. He observed that flies have four pairs of chromosomes, and that three of the pairs were identical in males and females. In females, the fourth pair had two identical chromosomes, now called X chromosomes. In males, the fourth pair had one X chromosome as well as a shorter chromosome, now known as a Y chromosome. Today, geneticists call the X and Y chromosomes as sex chromosomes. Sex Chromosomes and Autosomes The sex chromosomes contain genes that determine the gender of an individual. The remaining chromosomes that are not involved in determining the sex of an individual are called autosomes. autosomes Below you can see the karyotype of a human female, where there are two X chromosomes. The karyotype of a male has one X chromosome and one Y chromosome. In certain organisms, such as chickens and moths, males have two identical sex chromosomes, and females have two different sex chromosomes. Some organisms such as most plants and some fish lack sex chromosomes entirely. Sex Determination Sex chromosomes pair during meiosis I, like other homologous chromosomes. As meiosis proceeds, the paired chromosomes separate and move toward different cells. As a result, a sperm cell can receive either an X chromosome or a Y chromosome. Each egg receives an X chromosome, so the gender of a gamete

UNIT THREE: GENETICS AND BIOTECHNOLOGY (Text from Modern Biology, Holt, Rinehart, and Winston)

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depends on the sex chromosome the sperm that fertilizes it will have. This system of gender determination results in a one-to-one ratio of males to females. Each sperm and egg cell also receives a single copy of each autosome. In mammals, when an egg that carries an X chromosome is fertilized by a sperm with a Y chromosome, the resulting offspring has an XY pair and is male. The same thing happens with a sperm that has an X chromosome, except the offspring will be female. In a male mammal, the Y chromosome contains a gene called SRY for Sexdetermining Region Y, which codes for a protein that causes the gonads of an embryo to develop as testes. Since female embryos lack this gene, the gonads develop as ovaries. EFFECTS OF GENE LOCATION When Morgan was doing his research with fruit flies, one of the lab members noticed that single male fruit fly had white eyes instead of the normal red eyes. When Morgan crossed this white-eyed male with a normal red-eyed female, he found all the F1 offspring had red eyes, as expected. When he crossed a female and a male from this generation, the offspring had the expected 3 to 1 ratio of dominant to recessive, red-eyed to white-eyed. What was not expected was that all of the whiteeyed flies were male! XrY

F1

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Xr

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XR XRXr

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F1

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Y

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XRXR

XRY

XRY

XRXR XRXr

XR XRXr

XRY

F1

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Xr XRXr

XrY

UNIT THREE: GENETICS AND BIOTECHNOLOGY (Text from Modern Biology, Holt, Rinehart, and Winston)

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Sex-Linked Genes and Traits Based on his observation that all of the white-eyed flies were male, Morgan hypothesized that the gene for eye color is carried on the X chromosome and the Y chromosome lacks an allele for the eye-color gene. Thus, a Y chromosome cannot contribute an allele – only an X chromosome. This means that if a fly has the trait for having white eyes and is a male, there is no chance of having a dominant allele to give the fly the red eye color. Morgan called genes located on the X chromosome X-linked genes. He called genes located on the Y chromosome Y-linked genes. The term sexsex-linked trait refers to a trait coded for by an allele on a sex chromosome. Since the X chromosome is larger than the Y chromosome, there are more X-linked than Y-linked trait. Linked Genes Morgan and other geneticists hypothesized that if genes are inherited together, it must be because they are located on the same chromosome. Morgan studied two fly genes – one for body color and one for wing length – located on the same autosome. He first crossed a homozygous (GGLL) fly with another homozygous fly (ggll). Their offspring had the genotype GgLl and were gray with long wings. When he crossed that generation, they did not occur in the phenotypic ratio of 9:3:3:1, which meant they were not assorted independently, and therefore located on the same chromosome. He called pairs of genes that tend to be inherited together linked genes, genes and called a set of linked genes a linkage group. He also noticed that some offspring were unlike either parent, with gray bodies and short wings or with black bodies and long wings. He realized that mutations were too rare to have been the cause of these exceptions, and inferred that this was due to crossing-over, the exchange of pieces of DNA between homologous chromosomes.

UNIT THREE: GENETICS AND BIOTECHNOLOGY (Text from Modern Biology, Holt, Rinehart, and Winston) Chromosome Mapping The closer two genes are located on a chromosome, the more likely it is that they will cross-over together. Recombinants are offspring that do not look like their parents. The lower the recombination frequency, the closer the genes for those traits must lie on a chromosome, because they will cross over together. Researchers conduct breeding experiments and use the resulting data to prepare a chromosome map, map a diagram that shows the linear order of genes. Alfred H. Sturtevant, one of Morgan’s students, made the first chromosome map for flies. To prepare his map, he compared the frequency of crossingover for several genes. He defined one map unit as a frequency of crossing-over 1 percent. A map of the human X chromosome can be made using more recent techniques to map genes.

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Human X Chromosome

MUTATIONS A mutation is a change in the nucleotide-base sequence of a gene or DNA molecule. GermGerm-cell mutations occur in an organism’s gametes. These mutations do not affect an organism itself, but can be passed on to offspring. SomaticSomatic-cell mutations take place in an organism’s body cells and can therefore affect the organism, but not an organism’s offspring. They cannot be inherited. Lethal mutations cause death, often before birth. Some mutations result in phenotypes that are beneficial to an organism. Organisms with beneficial mutations will thus have an evolutionary advantage and have a greater chance of surviving and reproducing. Mutations provide the variations upon which natural selection acts, and can involve an entire chromosome or a single DNA molecule. Chromosome Mutations Chromosome mutations involve changes in the structure of a chromosome or the loss or gain of a chromosome. A deletion is the loss of a piece of a chromosome due to breakage. In an inversion, inversion a piece of the chromosome breaks off, flips around backward, and reattaches. In a translocation, translocation a piece of one chromosome breaks off and reattaches to a nonhomologous chromosome. In nondisjunction, nondisjunction a chromosome

UNIT THREE: GENETICS AND BIOTECHNOLOGY (Text from Modern Biology, Holt, Rinehart, and Winston)

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fails to separate from its homologue during meiosis. Thus, one gamete will have an extra copy of a chromosome while another will have no copies.

DELETION

INVERSION

TRANSLOCATION

Gene Mutations The substitution, addition, or removal of a single nucleotide is a point mutation, mutation which is a change that occurs within a single gene or other segment of DNA on a chromosome. In a substitution, substitution one nucleotide replaces another. If this occurs in a codon, then the amino acid that is supposed to be produced can be changed. In a deletion mutation, one or more nucleotides are lost. This loss can cause incorrect grouping of the remaining codons, called a frameshift mutation, mutation making all amino acids after the deletion change. Insertion mutations, mutations in which one or more nucleotides are added, can also result in a frameshift mutation.

UNIT THREE: GENETICS AND BIOTECHNOLOGY (Text from Modern Biology, Holt, Rinehart, and Winston)

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SECTION 1 REVIEW 1. Compare sex chromosomes with autosomes. 2. How does the inheritance of sex chromosomes result in approximately equal numbers of males and females among the offspring of fruit flies? 3. Explain why Morgan did not find white-eyed female D. melanogaster in the F2 generation when he crossed white-eyed males with red-eyed females. 4. How do sex linkage and crossing-over show that genes are found on chromosomes? 5. How can crossing-over between two alleles be used to map their locations on a chromosome? 6. Describe how nondisjunction can change chromosome number. CRITICAL THINKING 7. What cross could Morgan have performed to make the first white-eyed female fruit fly? 8. Three genes (A, B, and C) are on the same chromosome. However, only C and A are usually expressed together in the same phenotype. Explain this result and describe the relative positions of A, B, and C on the chromosome. 9. Chromosome mutations most often occur during nuclear division. Explain why this is a true statement.

SECTION TWO: HUMAN GENETICS INHERITANCE OF TRAITS Pedigrees A pedigree is a diagram that shows how a trait is inherited through several generations. In a pedigree, squares stand for males and circles stand for females. A filled symbol means that an individual has the trait or condition. An empty symbol means they do not. A horizontal line joining a male and a female indicate a mating, and a vertical line indicates offspring, arranged from left to right in order of birth. Roman numerals label different generations. Patterns of Inheritance By analyzing patterns of inheritance, the expression of genes over generations, biologists can learn about genetic diseases. If a trait is autosomal, it will appear in both sexes equally and if it is sex-linked, it is usually seen only in males.

UNIT THREE: GENETICS AND BIOTECHNOLOGY (Text from Modern Biology, Holt, Rinehart, and Winston)

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If a trait is autosomal dominant, every individual with the trait will have a parent with the trait. If it is recessive, an individual with the trait can have one, two, or neither parent exhibit the trait. If individuals with autosomal traits are homozygous dominant or heterozygous, then their phenotype will show the dominant characteristic. If individuals are homozygous recessive, their phenotype will show the recessive characteristic. Two people whoa re heterozygous carries of a recessive mutation will not show the mutation, but can produce offspring who are homozygous for the recessive allele. Individuals that have one copy of a recessive allele but do not have the disease are called carriers, carriers and can pass the allele to their offspring. GENETIC TRAITS AND DISORDERS Genetic disorders are diseases or disabling conditions with a genetic basis. Polygenic Inheritance Most human characteristics are polygenic: polygenic influenced by several genes. Polygenic characters show many degrees of variation. Eye color, height, skin color, and hair color are all polygenic characters. Complex Characters Many human conditions are complex characters – affected by both the environment and by genes. One example is skin color. Even if your genes determine that you have fair skin, if you go out in the sun for a long period of time, then your skin will become darker due to the environment (sun). Other complex characters play a role in diseases or conditions such as breast cancer, diabetes, stroke, heart disease, and schizophrenia. By identifying the environmental components that contribute to a disease, they can educate people in ways that minimize their risk of developing the disease. Multiple Alleles Genes with three or more alleles are said to have multiple alleles. alleles In humans, the ABO blood types are controlled by the alleles IA, IB, and i. The alleles IA and IB are codominant. In codominance, codominance both alleles are expressed in the phenotype of a heterozygote. Those two alleles are both dominant to the recessive i allele. Combinations of the three different alleles can produce four different blood types – A, B, AB, and O.

UNIT THREE: GENETICS AND BIOTECHNOLOGY (Text from Modern Biology, Holt, Rinehart, and Winston)

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Incomplete Dominance Sometimes, an individual displays a trait intermediate between the two parents, a condition known as incomplete dominance. dominance For example, the child of a straight-haired parent and a curly-haired parent would have wavy hair. X-Linked Traits Some complex characters are determined by X-linked genes, and a pedigree will show many affected males and no affected females. One form of colorblindness is a recessive X-linked disorder in which an individual cannot distinguish certain colors. Sex-Influenced Traits SexSex-influenced traits are involved in other complex characters. Males and females can show different phenotypes even when they share the same genotype. Sex-influenced traits are usually autosomal. For example, an allele that is dominant in males is recessive in females, due to higher levels of the hormone testosterone in men. Single-Allele Traits A single allele of a gene controls single-allele traits. Huntington’ Huntington’s disease is an autosomal dominant condition characterized by forgetfulness and irritability. DETECTING GENETIC DISEASE Many people with a family history of a genetic disease seek genetic screening before having children. Genetic screening is an examination of a person’s genetic makeup. Physicians can now detect more than 200 genetic disorders in the fetus. The technique called amniocentesis allows a physician to remove some amniotic fluid from the sac that surrounds the fetus. By examining chromosomes and proteins in the fluid, geneticists can analyze fetal cells for genetic disease. In chorionic villi sampling, sampling the physician takes a sample of the chorionic villi, cells derived from the zygote that grows between the mother’s uterus and placenta. Both procedures allow technicians to analyze fetal cells, chromosomes, proteins, and detect genetic disease.

UNIT THREE: GENETICS AND BIOTECHNOLOGY (Text from Modern Biology, Holt, Rinehart, and Winston)

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Genetic Counseling Many people with a family history of a genetic disease also undergo genetic counseling, counseling the process of informing a person or couple about their genetic makeup. This is a form of medical guidance that informs individuals about problems that may affect their offspring. TREATING GENETIC DISEASE Physicians treat genetic diseases in several ways. For many diseases, they can treat just the symptoms, such as PKU and cystic fibrosis. Gene Therapy Another level of treatment currently in development involves replacing the defective gene, called gene therapy, therapy which places a healthy copy of a gene into the cells of a person whose copy of the gene is defective. Gene therapy in which only body cells are altered is called somatic cell gene therapy. This contrasts with germ cell gene therapy, the attempt to alter eggs or sperm. This can affect future generations in unpredictable ways, therefore, it poses more risks and ethical issues. SECTION 2 REVIEW 1. A husband and wife have a son with cystic fibrosis. Their second child, a daughter, does not. Prepare a pedigree for this family. 2. Explain the difference between a polygenic character and a complex character. 3. A husband and wife have the ABO blood group genotypes IAIB and ii. What ABO blood types can their children have? 4. Compare Huntington’s disease with sickle cell anemia. 5. Describe the methods physicians use to detect genetic diseases in an unborn fetus. CRITICAL THINKING 6. A woman with cystic fibrosis marries a man who is heterozygous for cystic fibrosis. What is the likelihood that their children will have cystic fibrosis? 7. Why is colorblindness less common among females? 8. A man with blood type B marries a woman with blood type A. Their first child is blood type O. What is the probability that their next child will be blood type AB? Blood type B?

UNIT THREE: GENETICS AND BIOTECHNOLOGY (Text from Modern Biology, Holt, Rinehart, and Winston)

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CHAPTER HIGHLIGHTS Section 1: Chromosomes and Inheritance Genes reside on chromosomes. Sex chromosomes contain genes that determine an organism’s sex. The remaining chromosomes that are not directly involved in determining the sex of an individual are called autosomes. In mammals, an individual carrying two X chromosomes is female. An individual carrying an X and a Y chromosome is male. Genes found on the X chromosome are X-linked genes. A sex-linked trait is a trait whose allele is located on a sex chromosome. Because males have only one X chromosome, a male who carries a recessive allele on the X or Y chromosome will exhibit the sex-linked condition. Pairs of genes that tend to be inherited together are called linked genes. They occur close to each other on the same chromosome. The farther apart two genes are located on a chromosome, the more likely a cross-over will occur. Researchers use recombinant percentages to construct chromosome maps showing relative gene positions. Germ-cell mutations occur in gametes and can be passed on to offspring. Somatic-cell mutations occur in body cells and affect only the individual organism. Chromosome mutations are changes in the structure of a chromosome or the loss or gain of an entire chromosome. Gene mutations are changes in one or more of the nucleotides in a gene. Section 2: Human Genetics Geneticists use pedigrees to trace diseases or traits through families. Pedigrees reveal inheritance patterns of genes. A carrier has one copy of a recessive allele but does not express the trait. Polygenic characters, such as skin color, are controlled by two or more genes. Complex characters, such as height, are influenced by both genes and environment. Multiple-allele characters, such as ABO blood groups, are controlled by three or more alleles of a gene. The gene for colorblindness, an X-linked recessive, is found on the X chromosome. A sex-influenced trait, such as pattern baldness, is expressed differently in men than in women even if it is on an autosome and both sexes have the same genotype. Genetic screening examines a person’s genetic makeup and potential risks of passing disorders to offspring. Amniocentesis and chorionic villi sampling help physicians test a fetus for the presence of genetic disorders. Genetic counseling informs screened individuals about problems that might affect their offspring. Genetic disorders are treated in various ways. Among the treatments are symptomrelieving treatments and symptom-prevention measures, such as insulin injections for diabetes. Gene therapy is a type of treatment under development. IN gene therapy, a defective gene is replaced with a copy of a healthy gene. Somatic cell gene therapy alters only body cells. Germ cell gene therapy attempts to alter eggs or sperm.