SAT Inferences Question

Students in a biology class investigated why individual house mice (Mus musculus) can differ from one another in their susceptibility to cataracts in old age. The students compared wild-type mice and knockout mice, which are mice with specific genes deactivated, when mice of each type were raised in similar naturalistic environments and periodically tested for cataracts. Finding that knockout mice with the gene Aasdh deactivated tended to develop cataracts more frequently than did wild-type mice, the students concluded that differences in cataract frequency among house mice in nature are solely attributable to variations in the level of expression of Aasdh. Which finding, if true, would most directly weaken the students' conclusion?