Progeny Analysis
How are the vertical eyes and tooth traits inherited?

If the two genes showed independent assortment, we would hypothesize a 1:1:1:1 ratio of phenotypes from the testcross done.

Instead, most resemble the phenotypes of the two original parents (the normal round eyes and no tooth, and the double mutant vertical eyes and tooth). We must conclude that the genes do not assort independently.

The alternative is that they are linked; that is, the eye-shape and tooth phenotypes are usually inherited together because the genes for these two characters are near each other on the same chromosome; they are linked.