Question

To determine whether a particular trait is autosomal or sex-linked, scientists or breeders would perform reciprocal crosses. In a species of exotic bird, the wild-type glossy green is dominant over the yellow plumage. Assigning Ye⁺ to the wild type glossy green and Ye to yellow, show how you would perform such a cross (or crosses) to test the hypothesis that these plumage phenotypes in this exotic bird are sex-linked. What are the expected phenotypic ratios of the F2, after intercrossing the F1 in each case, if this trait is indeed sex linked?

Answer 1

Given:

Ye+ --> Glossy Green Plumage
Ye --> Yellow Plumage

Since birds have a ZZ-ZW system, where the male is the homogametic sex, having two Z chromosomes, and the female is the heterogametic sex, having one Z chromosome and one W chromosome. To determine if the plumage color is linked to the Z chromosome, the following series of crosses will be undertaken.

Cross 1: Female Bird with Glossy Green Plumage x Pure-breeding male with Yellow Plumage

F1 --> Female birds with Yellow plumage, Male birds with Glossy green plumage.
Genotypes: Zye W and Zye+ Zye

Cross 2: Female bird with Yellow plumage x Pure-breeding male with Glossy Green plumage

F1 --> Female birds with Glossy Green Plumage, Male birds with Glossy green plumage
Genotypes: Zye+ W and Zye+ Zye

Now, the F1 progenies above are the result of crossing pure-breeding lines of birds. The expected F2 progeny types after crossing them to one another are:

Cross 3:
Cross 1 F1 Female birds with Yellow plumage x Cross 1 F1 Male birds with Glossy green plumage

• 50% of males have Glossy green plumage
• 50% of males have Yellow plumage
• 50% of females have Glossy green plumage
• 50% of females have Yellow plumage

Cross 4:
Cross 2 F1 female with Glossy green plumage x Cross 2 F1 male with glossy green plumage

• 100% of males have Glossy green plumage
• 50% of female birds have yellow plumage
• 50% of female birds have glossy green plumage