Question

Imagine that the P gene lies on the Z chromosome, and controls the brightness of a bird’s plumage. The dominant wild-type Zpb allele produces bright plumage, while the recessive variant Zpd allele produces dull plumage. If you cross a pure-breeding bright male with a dull female, what proportion of genotypes/phenotypes do you expect to see in the F1 generation? If a male and female from the F1 generation mate, what genotypes/phenotypes do you expect to see in the F2 generation? When you give the phenotypes, give me sex and plumage status.

Answer 1

Answer: The sex chromosomes in bird are Z and W, as the gene ‘p’ is present on the Z chromosome, it is the sex linked trait.

In birds females are heteromorphic i.e. they have ZW chromosomes

Males are homomorphic i.e.they have ZZ chromosomes.

Part I) A pure breeding bright male will have the genotype ZpbZpb

The gamates produced by the male will be Zpb and Zpb

And a dull female will have the genotype ZpdW

The gamates produced by the female will be Zpd and W

The cross would be as follows

gamates	Zpb	Zpb
Zpd	ZpbZpd Bright male	ZpbZpd Bright male
W	ZpbW Bright female	ZpbW Bright female

Hence in F1 all the phenotypes will be with bright plumage.

Part II) If F1 male and females are crossed then

Bright male’s genotype is ZpbZpd

The gamates produced by the male will be Zpb and Zpd

And a bright female’s genotype will be ZpbW

The gamates produced by the female will be Zpb and W

The cross would be as follows

gamates	Zpb	Zpd
Zpb	ZpbZpb Bright male	ZpbZpd Bright male
W	ZpbW Bright female	ZpdW Dull female

The phenotypic ratio is 3(bright):1(dull)