The generalization that when one sex is absent, rare, or sterile, in the offspring of two different animal races or species, that sex is the heterogametic sex. The Haldane rule is known to apply for various species of mammals, birds, and insects. In Drosophila and Mus, the X and Y chromosomes interact during spermatogenesis, with the Y repressing the transcription of certain X-linked loci. Presumably, when the X and Y chromosomes are from different species, such regulation does not take place and sterility results. Thus, the Haldane rule may be explained by the nonharmonious interaction of X- and Y-linked fertility genes in the hybrid. See Chronology, 1922, Haldane.
Subjects: Genetics and Genomics.