As a result of linkage studies, genes linked together and thus not showing independent assortment are grouped together in a linkage group. However, number of such linkage groups in a particular species was found to correspond to the haploid number of chromosomes, indicating that genes linked together are situated on the same chromosome. Drosophila melanogaster has four linkage groups which correspond to n = 4. In maize, there are 10 linkage groups (n = 10) and in barley there are only seven linkage groups (n = 7).
Complete linkage in Drosophila males
While using Drosophila flies in linkage studies, Bridges discovered that no crossing over took place in male individuals. For instance, if F1 (AB/ab) is used as male and testcrossed with ab/ab as female, only two types of progenies, i.e., AB/ab and ab/ab, will be obtained in the next generation.
For this reason, in preparation of linkage maps, particularly in Drosophila melanogaster,
hybrid is always used as a female parent. On the other hand, if only linkage groups are to be established, the presence of complete linkage in Drosophila melanogaster
males and in the heterogametic sex in a few other species (e.g. female silkworm) offers a special advantage. For instance, if F1
is used as male, linkage will be demonstrated by simple absence of crossovers in the testcross progeny. However, there is no suitable explanation available for the absence of crossing over in male Drosophila
flies. However, this phenomenon is not observed in all species of Drosophila.