Re: Why doesn't crossing over occur in the male germ cells of Drosophila melanogaster?

Drosophila males are unusual in this regard; the total absence of crossing over in one sex occurs in only a limited number of species. In others, crossing over or recombination occurs at more or less the same frequency in both males and females.

The absence of crossing over during meisois in Drosophila males results from an unusual prophase I in which synaptonemal complexes do not form. These protein complexes are required for the accurate pairing of homologous chromosomes. Since these complexes do not form during the early stages of meiosis, crossing over does not occur.

As an aside, there is also a difference in the frequency of recombination in human males and females. In general, women show higher recombinant frequencies for the same loci than do men.