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6.01 One of the reasons that Mendel’s principles were hard for biologists in his day to comprehend was that the details of cell and nuclear structure were largely unknown then. How did the discovery of chromosomes in the late 1880s make Mendel’s ideas more believable?
6.01 One of the reasons that Mendel’s principles were hard for biologists in his day to comprehend was that the details of cell and nuclear structure were largely unknown then. How did the discovery of chromosomes in the late 1880s make Mendel’s ideas more believable?
Answer: In Mendel’s day, the details of nuclear structure had not been described. When chromosomes were discovered (during meiosis) to behave just as Mendel’s “particles” should, it was much easier to related the cytological details of chromosomes to Mendel’s proposals.
Answer: In Mendel’s day, the details of nuclear structure had not been described. When chromosomes were discovered (during meiosis) to behave just as Mendel’s “particles” should, it was much easier to related the cytological details of chromosomes to Mendel’s proposals.
6.02 Wild-type Drosophila flies have red eyes, but a certain mutant type has white eyes. This trait is known to be controlled by a single pair of alleles. A cross between a red-eyed heterozygous female fruit fly and red-eyed male fruit fly yielded 94 red-eyed and 30 white-eyed progeny.
6.02 Wild-type Drosophila flies have red eyes, but a certain mutant type has white eyes. This trait is known to be controlled by a single pair of alleles. A cross between a red-eyed heterozygous female fruit fly and red-eyed male fruit fly yielded 94 red-eyed and 30 white-eyed progeny.
(a) Using a chi-square test check to see if these data are consistent with the hypothesis that white eyes is caused by a recessive sex-linked allele.
(a) Using a chi-square test check to see if these data are consistent with the hypothesis that white eyes is caused by a recessive sex-linked allele.
Answer:
Answer:
The calculated chi-square value = 0.043, which is less than the critical value of 3.84 (df=1). The data are consistent with the hypothesis of sex-linked recessive.
The calculated chi-square value = 0.043, which is less than the critical value of 3.84 (df=1). The data are consistent with the hypothesis of sex-linked recessive.
(b) 56 of the red-eyed flies were females. Is this consistent? (Use another chi-square test)
(b) 56 of the red-eyed flies were females. Is this consistent? (Use another chi-square test)
Answer:
Answer:
The calculated chi-square value = 2.13, which is less than the critical value of 3.84 (df=1). The data are consistent with the predicted female to male ratio among the wild type (red eyed) offspring.
The calculated chi-square value = 2.13, which is less than the critical value of 3.84 (df=1). The data are consistent with the predicted female to male ratio among the wild type (red eyed) offspring.
6.03 Thomas Hunt Morgan’s research with sex-linked traits in Drosophila led to his conclusion that genes are actually located on chromosomes. Explain how he was able to conclude this. That is, how does the existence of sex-linked traits confirm that chromosomes carry the alleles?
6.03 Thomas Hunt Morgan’s research with sex-linked traits in Drosophila led to his conclusion that genes are actually located on chromosomes. Explain how he was able to conclude this. That is, how does the existence of sex-linked traits confirm that chromosomes carry the alleles?
Answer: It was clear from microscopic observations that male flies had only one X chromosome while females had two. The results of crosses between red-eyed and white-eyed flies were explained perfectly if one could assume that females had two copies of the eye color gene while males only had one. Thus there was a clear connection between the phenotypic results observed from the cross and the chromosomes actually carried by the parents.
Answer: It was clear from microscopic observations that male flies had only one X chromosome while females had two. The results of crosses between red-eyed and white-eyed flies were explained perfectly if one could assume that females had two copies of the eye color gene while males only had one. Thus there was a clear connection between the phenotypic results observed from the cross and the chromosomes actually carried by the parents.
6.04 A certain rare human disease is caused by a dominant sex-linked allele. Would you expect this disease to be more common among males or females? Explain your answer.
6.04 A certain rare human disease is caused by a dominant sex-linked allele. Would you expect this disease to be more common among males or females? Explain your answer.
Answer: The disease should be more common among females because females have tow chances to get the allele (they have 2 X-chromosomes). Males only have one X. Looking at it another way, males can only get the disease when their mother has it; females can inherit it from either parent.
Answer: The disease should be more common among females because females have tow chances to get the allele (they have 2 X-chromosomes). Males only have one X. Looking at it another way, males can only get the disease when their mother has it; females can inherit it from either parent.
6.05 The pedigree chart below shows the pattern of inheritance for a certain human disease, known to be caused by a single gene. Open symbols represent normal individuals, filled symbols represent affected individuals.
6.05 The pedigree chart below shows the pattern of inheritance for a certain human disease, known to be caused by a single gene. Open symbols represent normal individuals, filled symbols represent affected individuals.
(a) Which of the following is the most likely mode of inheritance of this disease: autosomal dominant, autosomal recessive, sex-linked dominant, sex-linked recessive?
(a) Which of the following is the most likely mode of inheritance of this disease: autosomal dominant, autosomal recessive, sex-linked dominant, sex-linked recessive?
Answer: The most likely is sex-linked dominant. The trait appears in every generation and daughters inherit the trait from their fathers.
Answer: The most likely is sex-linked dominant. The trait appears in every generation and daughters inherit the trait from their fathers.
(b) Which of these modes of inheritance is possible?
(b) Which of these modes of inheritance is possible?
Answer: All the following are possible: autosomal dominant, autosomal recessive, sex-linked dominant.
Answer: All the following are possible: autosomal dominant, autosomal recessive, sex-linked dominant.
(c) Which of these modes of inheritance is not possible?
(c) Which of these modes of inheritance is not possible?
Answer: Sex-linked recessive is not possible because the affected daughter in the second generation would have to be homozygous recessive. If this were the case, all of her sons would be affected, but she has an unaffected son.
Answer: Sex-linked recessive is not possible because the affected daughter in the second generation would have to be homozygous recessive. If this were the case, all of her sons would be affected, but she has an unaffected son.
6.06 In Drosophila there is an autosomal gene that determines body color. Ebony body (e) is recessive to the wild type (gray) body(e+). A second gene, which is X-linked, controls wing size. Normal wing size (m+) is dominant to the mutant miniature wings(m). A female with miniature wings that is heterozygous for the body color gene is mated with an ebony body male (his wings are normal).
6.06 In Drosophila there is an autosomal gene that determines body color. Ebony body (e) is recessive to the wild type (gray) body(e+). A second gene, which is X-linked, controls wing size. Normal wing size (m+) is dominant to the mutant miniature wings(m). A female with miniature wings that is heterozygous for the body color gene is mated with an ebony body male (his wings are normal).
(a) What are the genotypes of the parents?
(a) What are the genotypes of the parents?
Answer: Xm Xm e+e x Xm+ Y e e
Answer: Xm Xm e+e x Xm+ Y e e
(b) Using a Punnett square or the forked-line method, determine the expected proportions of each possible progeny phenotype. Since there is a sex-linked gene here, be sure to include consider sex as part of the phenotype.
(b) Using a Punnett square or the forked-line method, determine the expected proportions of each possible progeny phenotype. Since there is a sex-linked gene here, be sure to include consider sex as part of the phenotype.
Answer:
Answer:
6.07 In chickens, males are the homogametic sex, ZZ, and females are the heterogametic sex, ZW. In a cross between ZAW and ZAZa, where the superscripts indicate the dominant and recessive sex-linked alleles, what proportions of phenotypes would you expect in the progeny? Would the proportions be the same in both sexes?
6.07 In chickens, males are the homogametic sex, ZZ, and females are the heterogametic sex, ZW. In a cross between ZAW and ZAZa, where the superscripts indicate the dominant and recessive sex-linked alleles, what proportions of phenotypes would you expect in the progeny? Would the proportions be the same in both sexes?
Answer:
Answer:
 The ratios are not the same for males and females. In the ZW system, the inheritance patterns are similar to in the XY system, but the outcomes for males and females are reversed.
 The ratios are not the same for males and females. In the ZW system, the inheritance patterns are similar to in the XY system, but the outcomes for males and females are reversed.
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