What happens if you cross two flies together that show variation in not one gene, but two?

The answer depends on whether the two genes are located on the same chromosome or on different ones.

As an example of the latter case, consider the consequences of crossing two flies that are defective in their Adhand rosygenes. The rosygene, located on chromosome 3, encodes the enzyme xanthine dehydrogenase or XDH. XDH is involved in the synthesis of some of the eye pigments of Drosophila. When a fly carries two defective rosy genes, its eye color is different than the eye color of wild type ("normal") flies.

As you know, the Adhgene is located on chromosome 2. What germ cells will be formed by a fly that carries one defective Adh gene and one defective rosygene? Think about the answer to this question before you go on to the next page.