The main features of these deviations, the causes of which have never been determined, are as follows. Moreover, the loci affected vary from cross to cross, so that markers conform to Mendelian inheritance at least part of the time. The causes appear to be biological rather than technical. Deviations are evident during larval development but are especially marked at the juvenile or adult stages.
Deviations occur in random-bred progenies, but they are particularly striking in inbred families.
Such widespread distortions of Mendelian inheritance could be caused by selection against numerous deleterious mutations or combinations of genes linked to the observed markers. Tissues from the two hybrid parents, the four grandparents and a variety of other relatives for each of the nine segregating populations were available for microsatellite analyses. Nevertheless, the genealogy of these null alleles can be traced in every instance. Thus, we confirm the prediction that there should be little segregation distortion in early larvae. We then sampled the same hybrid families at the juvenile stage to confirm that segregation ratios progressively distort through development as the result of the hypothesized selection against linked deleterious genes or gene-combinations.