Chapter 2

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Apr 3, 2024

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Module 3 (Chapter 2) Homework: 1. Two alleles of gene C control hair color in horses: C1 and C2. Horses homozygous for allele C1 are red, heterozygotes are yellow, and C2 homozygotes are cream. What type of allele interaction is described?  Incomplete dominance 2. If a trait is controlled by two different genes, a possible interaction between alleles of these genes can be called:  Epistasis 3. Curly-winged flies mate with pure-breeding straight-winged (wild-type) flies. The curly-winged F1 mate with each other to produce an F2 generation that consists of 160 flies with curly wings and 80 with straight wings. What can you infer from this observation?  The dominant curly wing allele is also a recessive lethal. ***lethal alleles are when there is a 2:1 ratio instead of a 3:1 ratio; 80 is half of 160 so this is a 2:1 ratio (this is recessive lethal because the progeny did not die that displayed the dominant curly-winged gene, but if dominant lethal, then the progeny with the dominant curly-winged gene would have died/eventually died). 4. What can explain the phenomenon where in different individuals, a particular genotype might give rise to different phenotypes?  Penetrance and expressivity ***The term penetrance is used to describe a proportion of individuals with a particular genotype who showed the expected phenotype; the term expressivity refers to the degree or intensity with which a particular genotype is expressed in a phenotype (shown in the genotype for cancer retinoblastoma). 5. Two alleles of gene C control hair color in horses: C1 and C2. Horses homozygous for allele C1 are red, heterozygotes are yellow, and C2 homozygotes are cream. In the offspring of mating’s’ between heterozygotes, what phenotypic ratio is expected?  1 red: 2 yellow: 1 cream

***Because this is mating between heterozygotes & we do not know if C1 or C2 are recessive or dominant traits, we know that the more common progeny will be the heterozygote phenotype (yellow) and the homozygous phenotypes will be expressed equally to each other (when crossing two heterozygotes – you will have four results showing the different phenotypic outcomes). 6. If a trait is controlled by two codominant alleles of one gene, what phenotypic ratio is expected in the offspring of a mating of two heterozygotes?  1:2:1 7. Which genotypic ratio indicates a recessive lethal allele when two heterozygotes are mated?  2:1 8. ABO blood types demonstrates which of the following inheritance patterns? (Check all that apply)  Pleiotropy  Codominance 9. A particular flower can be blue, red, or white. A pure-breeding red- flowered plant is crossed with a pure-breeding white-flowered one. The F1 are then crossed to produce an F2 generation. Which of the following phenotypic ratios in the F2 indicate that flower color in these two strains is controlled by two genes? RW Rw rW rw RW RRWW (R) RRWw (R) RrWW (R) RrWw (R) Rw RRWw (R) RRww (B) RrWw (R) Rrww (B) rW RrWW (R) RrWw (R) rrWW (W) rrWw (W) rw RrWw (R) Rrww (B) rr Ww (W) rrww (B) R = red W = white B = blue  9:3:4

10. If a gene for a trait is monomorphic in a population and two random individuals mate, what would be the most likely phenotypic ratio for that trait in the offspring?  1 11. What genetic phenomenon might a 2:1 phenotypic ratio indicate?  Recessive lethality 12. A particular flower can be purple, blue, red, or white. Plants from two different pure-breeding strains of white-flowered plants are crossed and the F1 are then crossed to produce an F2 generation. What might a 9:7 phenotypic ratio in the F2 indicate about the genes that control flower color?  Two genes with reciprocal recessive epistasis. ***Reciprocal recessive epistasis: when homozygous (the pure-breeding strains), the recessive allele of each gene masks the dominant allele of the other gene. 13. A particular flower can be purple, blue, red, or white. A pure-breeding purple-flowered plant is crossed with a pure-breeding white-flowered plant and the F1 are then crossed to produce an F2 generation. What might a 9:3:4 phenotype ratio in the F2 indicate?  Recessive epistasis ***With recessive epistasis: when homozygous (the pure-breeding strains), the recessive allele of one gene mask both alleles of another gene. 14. Which ratio in the F2 of a cross between two pure-breeding strains would indicate that a phenotype is controlled by more than one gene?  9:3:3:1 ***Additive gene interaction: when there are four distinct F2 phenotypes. 15. Some complex traits are determined by many interacting genes, each of which may have several alleles. If such a trait is measured in a population, what phenotypic pattern is expected?

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