Final answer:
Drosophila male and female strains can crossbreed due to their hemizygous nature and sexual selection behaviors, along with homologous recombination that reshuffles alleles during meiosis.
Step-by-step explanation:
The mechanism that allows Drosophila melanogaster male and female strains to crossbreed and have fertile offspring is rooted in their genetic makeup and the behavior of X-linked genes during mating. In these fruit flies, males are XY and females are XX, so the males are hemizygous for X-linked traits, such as eye color. When a P generation male with the white-eye phenotype (XwY) mates with a homozygous red-eyed female (XWXW), the F1 generation will have all red-eyed offspring since the red eye allele (XW) is dominant.
Crossbreeding is further facilitated by sexual selection, where flies will tend to choose optimal mates, as well as homologous recombination, which can reshuffle alleles between the X chromosome and its homolog during meiosis.