Chapter 1. Drosophila Genetics II

Drosophila Genetics II

by Laurel Hester, Candace McGuinness, Brent Fuller, and Kirk A. Stowe

Now that you know a bit more about Mendelian genetics and X-linked crosses, let’s discuss your two crosses again, starting with the known X-linked cross. Recall that the parents for the initial cross performed at the biological supply company (P) were homozygous recessive white-eyed females and wild-type red-eyed males. This can be written as: X^wX^w × X^WY. Sex-linked crosses are written this way to remind us that these genes are located on a sex-determining chromosome.

Two weeks ago you saw that in the F₁ generation all males had white eyes and all females had red eyes. Now do the Punnett square for this cross to figure out why this was so.

F₁ Flies

In the F₁ generation for this cross, why did all males have white eyes and all females have red eyes?

Do another Punnett square to predict the phenotypic ratio of the F₂ offspring you will observe today. This time you need to write in the parental genotype based on your F₁ Punnett square.

F₂ Flies

Expected Phenotypic Ratio

___ red-eyed females: ___ white-eyed females: ___ red-eyed males: ___ white-eyed males

Our hypothesis for the sex-linked cross is that our observed phenotypic ratio will match the expected phenotypic ratio determined by the above Punnett square.