Important Terms

codominance (p. 103) lethal allele (p. 104) multiple alleles (p. 105) gene interaction (p. 107) epistasis (p. 108) epistatic gene (p. 108) hypostatic gene (p. 108) complementation test (p. 114)

complementation (p. 115) sex-influenced characteristic

sex-limited characteristic (p. 116) cytoplasmic inheritance (p. 118)

genetic maternal effect (p. 119)

genomic imprinting (p. 120) anticipation (p. 121) norm of reaction (p. 121) temperature-sensitive allele (p. 122) phenocopy (p. 123) discontinuous characteristic (p. 123)

continuous characteristic

(p. 123) quantitative characteristic (p. 123)

polygenic characteristic (p. 123) pleiotropy (p. 123) multifactorial characteristic (p. 123)

Worked Problems

1. The type of plumage found in mallard ducks is determined by three alleles at a single locus: MR, which codes for restricted plumage; M, which codes for mallard plumage; and md, which codes for dusky plumage. The restricted phenotype is dominant over mallard and dusky; mallard is dominant over dusky (MR > M > md). Give the expected phenotypes and proportions of offspring produced by the following crosses.

We can determine the phenotypes and proportions of offspring by (1) determining the types of gametes produced by each parent and (2) combining the gametes of the two parents with the use of a Punnett square

(a) Parents

Gametes

M Rmd

Mmd

restricted

mallard

y2 restricted, y2 mallard

(b) Parents

Gametes y2 restricted, y2 mallard

M Rmd X Mmd

M RM restricted

M Rmd restricted

Mmd mallard

Gametes

M Rmd X MRM

M RM R restricted

M RM restricted

M Rmd restricted

Mmd mallard

3/4 restricted, y4 mallard

(d) Parents

Gametes

3/4 restricted, y4 mallard

MRM X Mmd

M RM restricted

M Rmd restricted

MM mallard

Mmd mallard

y2 restricted, y2 mallard

2. A homozygous strain of yellow corn is crossed with a homozygous strain of purple corn. The F1 are intercrossed, producing an ear of corn with 119 purple kernels and 89 yellow kernels (the progeny).

(a) What is the genotype of the yellow kernels?

(b) Give a genetic explanation for the differences in kernel color in this cross.

(a) We should first consider whether the cross between yellow and purple strains might be a monohybrid cross for a simple dominant trait, which would produce a 3:1 ratio in the F2 (Aa X Aa : 3/4 A_ and 1/4 aa). Under this hypothesis, we would expect 156 purple progeny and 52 yellow progeny:

y2 restricted, y4 mallard, y4 dusky

Phenotype purple yellow total

Genotype

0 0

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