Lecture 25: Altered Mendelian ratios: epistasis, pleiotropy.
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 antigen (FUT1*O). 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. Dominant inhibition may give rise to a 12:3:1 ratio or a 13:3 ratio. Explain how these two different ratios can arise and cite an example of each.
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
e. Incomplete penetrance
a. 9:3:3:1
b. 6:3:3:2:1:1
c. 4:2:2:2:2:1:1:1:1
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.
e. Recessive epistasis.
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. 12:3:1
c. 12:4
d. 9:7
e. 15:1
f. 9:6:1
g. 9:4:3
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 the phenotype. You should be able to come up with two totally different answers for this question.
14. Distinguish between penetrance and expressivity in a manner that demonstrates that you know what both are and how they differ.
15. What mechanism is responsible for black ears and a white body coat on a Himalayan rabbit?
16. A true-breeding eyeless fly (ey/ey) is mated with a true-breeding brown eyed fly (bw/bw). Assume that both mutations are recessive and are located on different autosomes and that both have 100% penetrance (which is not always true for eyeless in real life).
a. What will be the phenotype of the F1 progeny?
b. What will be the phenotypic distribution when the F1 progeny are test corssed?
c. What will be the phenotypic distribution of the F2 progeny of the original cross?
d. What fraction of the brown-eyed F2 flies are heterozygous for the eyeless allele?
e. What fraction of the wild-type F2 flies are heterozygous for the eyeless allele?
17. Eyeless (ey), scarlet-eyed (st), and brown-eyed (bw) are unlinked autosomal recessive loci in Drosophila. The F2 generation of a cross of brown-eyed and scarlet-eyed flies included a limited number of white-eyed flies. When these were mated with one-another, they produced a true breeding white-eyed strain. That strain was crossed with a true-breeding strain of eyeless flies (which can be assumed to be wild-type at all other loci).
a. What is the genotype of the true-breeding white-eyed flies (include all three loci in your description)?
b. What is the genotype of the true-breeding eyeless strain of flies (again, include all three loci)?
c. What will be the genotype of the F1 generation of the cross between the white-eyed strain and the eyeless strain?
d. What will be the phenotype of the F1 generation of the cross between the white-eyed strain and the eyeless strain?
e. Use a forked line approach to determine the ratios of all possible phenotypes in the F2 generation of that cross.
18. Cite an example in which interactions among alleles at two different genetic loci causes four different phenotypic manifestations that all affect the same organ or tissue.
19. An F1 hybrid between dwarf peas with purple flowers and tall peas with white flowers is tall and has purple flowers. Can this be described as complementation? Defend your answer.
20. Under what circumstances can a dominant phenotypic trait skip a generation?
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