Haldane's rule

Haldane's rule is an observation in the early stage of speciation. It was formulated in 1922 by the British evolutionary biologist J.B.S. Haldane. Haldane's Rule states that in a species hybrid if only one gender is inviable or sterile, this is more likely to affect the heterogametic sex, that is, the sex that has two different, rather than two identical, sex chromosomes (in mammals, for example, this would be the male, since males have XY sex chromosomes, females have XX).

Haldane himself described the rule as:

(F1 means first generation of cross mating. F2 however would mean offspring produced by crossing among F1 offsprings.)

Haldane's rule applies to the vast majority of heterogametic organisms examined, including when two species, where secondary contact in an area of sympatry results in incomplete reproductive isolation, form hybrids after allopatric speciation has occurred. The rule include both male heterogametic (XY or XO-type sex determination, such as found in mammals and Drosophila) and female heterogametic (ZW-type sex determination, such as found in birds and Lepidoptera), and some dioecious plants such as Silene. It appears to be a general pattern associated with heterogamety.