Chapter 1. Drosophila Genetics II

Drosophila Genetics II

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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: XwXw × XWY. 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.