Lecture 8: Altered Mendelian ratios: epistasis, pleiotropy.
Please note that these questions use C/c for the mouse coat color locus that causes the homozygous recessive (c/c) to be white (not pigmented). This locus was generically designated B/b in the textbook in a generalized discussion of gene interactions involving two loci. Also, some of the topics incorporated into the questions for this lecture were introduced in lecture 7.
These questions have been rearranged substantially from the 1997 list that was initially posted. Questions on topics that will be covered in the first examination have been moved to the first half of the list. Those that wil not be dealt with until the second examination have been moved to the second half. Questions about continuous variaiton and polygenic inheritance have been moved to lecture 9. Question 12 is from the questions for 1997 lecture 6.
1. A man and a woman who both have blood types AB and MN marry and have a child. Construct a Punnett square to determine the possible blood types of the child and the probability of each.
2. A woman has blood types AB and MN.
a. Without any knowledge of the father's blood type, what possible blood types can her children have?
b. If the father is type A and type M, what are the possibilities for the children. Include in your answer a discussion of possible areas of uncertainty about the expected blood types of the children.
3. A couple both have blood type AB and are both heterozygous for the mutation that causes absence of H substance (h). What will the distribution of ABO blood types be for their children?
4. Explain the role played by the agouti locus in wild type mice and the ways that its function is altered (if at all) in each of the following. Also identify those cases in which you need more information to provide an answer.
a. A wild-type gray mouse
b. A yellow mouse
c A solid color black mouse
d. A solid color brown mouse
e. A cinnamon mouse
f. A white mouse
5. Assume that you are dealing only with the agouti (A) and coat color (C) loci in this problem and that the mice are wild type at all other loci. A white mouse of unknown genotype is crossed with an agouti mouse of unknown genotype. Half of their progeny are white, one fourth are black, and one fourth are agouti.
a. What is the genotype of the agouti parent?
b. What is the genotype of the white parent?
c. What possible genotypes can the white progeny have?
d. Two of the agouti progeny are mated. Use a forked line approach to determine the theoretical phenotypic ratios for their progeny.
e. One of the black progeny is mated with one of the agouti progeny. Use the forked line approach to determine the theoretical phenotypic ratios for their progeny.
f. One of the white progeny is mated with one of the black progeny. Their offspring include some agouti mice. What is the expected phenotypic ratio among the progeny of pairs whose offspring include agouti.
6. Describe the difference between discontinuous and continuous variation.
7. Briefly define each of the following, including a description of how you would verify that it was occurring: .
a. Pleiotropy8. What circumstances would lead to each of the following phenotypic distributions in the F2 generation? .
b. Epistasis
c. Dominant lethal. .
d. Altered phenotypic ratio
a. 9:3:3:1
b. 9:3:4
c. 6:3:3:2:1:1
d. 2:1
e. 3:1
9.. Briefly define each of the following, including a description of how you would verify that it was occurring: .
a. Dominant inhibition of gene expression10. White leghorn chickens have normal pigment genes, but are white because of a dominant color inhibiting gene (I). White Plymouth rock chickens are white because they are homozygous for a recessive loss of pigmentation gene (c) that behaves much like the coat color gene (C/c) in mice. The genes are unlinked and neither is sex-linked. A true-breeding white leghorn (II CC) is crossed with a true-breeding white Plymouth Rock chicken (ii cc).
b. Codominance
c. Complementary gene action.
d. Duplicate gene action.
a. What is the genotype of the F1 progeny?11. In summer squashes, there are three common fruit colors, white, yellow, and green. White color is caused by a dominant gene (W) that is epistatic over any other color combinations. In crosses between yellow and green, yellow is found to be dominant, due to a dominant gene (Y) that is not linked to W. A homozygous white squash plant that carries the dominant yellow allele (WW YY) is crossed with a green squash plant (ww yy). .
b. What is the phenotype of the F1 progeny?
c. Draw a Punnett square for the F2 whose individual squares are large enough to enter both the genotype and the phenotype of all possible combinations. Insert the genotypes and the phenotypes into the squares.
d. What is the phenotypic ratio of the F2 progeny of the original cross?
a. What is the genotype of the F1 progeny?12. What circumstances would lead to each of the following phenotypic distributions in the F2 generation? .
b. What is the phenotype of the F1 progeny?
c. Draw a Punnett square for the F2 whose individual squares are large enough to enter both the genotype and the phenotype of all possible combinations. Insert the genotypes and the phenotypes into the squares.
d. What is the phenotypic ratio of the F2 progeny of the original cross? .
a. 9:3:3:1
b. 9:3:4
c. 15:1
d. 9:7
e. 12:4
f. 9:6:1
13. Explain how two parents who are both afflicted with a particular non-lethal phenotype, such as deafness can have children who are not afflicted. You may make any assumptions you wish about the mode(s) of inheritance of thephenotype. You should be able to come up with two totally different answers for this question.
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