It very frequently occurs in interspecific hybridization that the members of one sex are affected more by reduced fertility or viability. Haldane’s rule, formulated in 1922 by J.B.S. Haldane, states that, in this case, the heterogametic sex, i.e. the sex whose cells contain both types of sex chromosomes, is affected far more frequently or to a far greater degree (Haldane 1922). In cases of the Drosophila type (e.g, in mammals), females with sex chromosomes XY are affected more than males with sex chromosomes XX; in cases of the Abraxas type (e.g. in birds), on the other hand, females with sex chromosomes ZW are affected more than males with sex chromosomes ZZ (Civetta & Singh 1999). The validity of Haldane's rule has been repeatedly demonstrated for a wide range of species belonging to various taxa. Of 223 known cases of sterility affecting only the members of one sex, 99% of them corresponded to Haldane’s rule. In relation to the inviability of hybrids, 90% of 115 described cases obeyed Haldane’s rule (Turelli 1998). This survey included only cases of complete sterility or complete inviability of one sex; if we were to also consider partial sterility and reduced viability of hybrids of one sex, the number of known cases would increase many fold; however the percentage of cases obeying Haldane’s rule would remain roughly the same.
It is apparent that several mechanisms are simultaneously valid here, of which the best known, i.e. the dominance hypothesis, faster male hypothesis, hypothesis of greater resistance of oocyte formation, recessive gene hypothesis, somatic mutation hypothesis and ultraselfish genetic element hypothesis – will be described below. This relatively unimportant phenomenon of lower viability and fertility of hybrid members of the heterogametic sex will be discussed in greater detail because this can be a key to understanding an extremely important phenomenon – speciation dependent on genetic incompatibility. In this connection it is recommended that the reader study more carefully the sections on the dominance hypothesis, the faster male hypothesis and the somatic mutation hypothesis. Unless explicitly stated otherwise, the relevant mechanisms are applicable similarly as for the Drosophila and Abraxas types. For simplification, where possible, the situation will be described for the Drosophila type.