What is Haldane's rule in simple terms?

If only one sex of a hybrid between two species has problems, it is the sex that carries two different kinds of sex chromosome.

The leading explanation, dominance theory, holds that variants causing hybrid incompatibility are mostly recessive, so their effects show up in the sex with only one X or Z chromosome.

Haldane's Rule

🔬 Natural Sciences By CampusDown Wiki Editorial Team Last updated October 27, 2026

Haldane's rule is the generalization proposed by the British geneticist J. B. S. Haldane in 1922 that when, in the first-generation hybrids between two species, one sex is absent, rare or sterile, that sex is the heterogametic sex, carrying two different sex chromosomes (XY males in mammals and Drosophila, ZW females in birds and butterflies).

Contents

1. Content of the rule and examples
2. Genetic explanations
3. Tests and modern research
4. FAQ
5. References

Content of the rule and examples

Reviewing data on interspecific hybrids in many animals, Haldane (1922) found that when sterility or reduced viability affects only one sex, it is almost always the heterogametic sex. Because males are heterogametic in Drosophila and mammals while females are heterogametic in birds, butterflies and moths, the affected sex is reversed between these groups. Coyne and Orr (1989, 2004) listed the rule as one of the two rules of speciation and reported that most Drosophila hybrid cases obey it. Laurie (1997), reviewing 75 years of work, described it as one of the most consistent speciation patterns in animals.

Genetic explanations

The leading explanation is the dominance theory. Turelli and Orr (1995, 2000) showed that if variants causing genetic incompatibilities between species are mostly recessive, their effects are unmasked in the heterogametic sex, which carries only one X or Z chromosome. Wu and Davis (1993) added a faster-male hypothesis, in which sexual selection on males makes male reproductive genes evolve quickly, and argued that Haldane's rule is a composite of several causes. A faster-X effect, in which sex chromosomes contribute disproportionately to hybrid incompatibility, has also been proposed, and Presgraves (2008) reviewed the evidence for it in Drosophila.

Tests and modern research

Tao and Hartl (2003) genetically dissected hybrid incompatibility factors between Drosophila species and showed that their degree of dominance and pattern of accumulation vary widely. Price and Bouvier (2002) supported the rule in birds by finding that viability and fertility of hybrid females decline before those of males. Schilthuizen, Giesbers and Beukeboom (2011) and Delph and Demuth (2016) reviewed cases and exceptions across plants and animals and concluded that the dominance theory has the broadest support. The rule serves as a key clue to how natural selection and genetic drift build reproductive isolation.

FAQ

What is Haldane's rule in simple terms?: If only one sex of a hybrid between two species has problems, it is the sex that carries two different kinds of sex chromosome.
Why does it happen?: The leading explanation, dominance theory, holds that variants causing hybrid incompatibility are mostly recessive, so their effects show up in the sex with only one X or Z chromosome.

Sources reviewed: 2026-10-26

References

Haldane, J. B. S. (1922). Sex ratio and unisexual sterility in hybrid animals. Journal of Genetics, 12(2), 101–109.
Coyne, J. A., & Orr, H. A. (1989). Two rules of speciation. In D. Otte & J. A. Endler (Eds.), Speciation and Its Consequences (pp. 180–207). Sunderland, MA: Sinauer.
Coyne, J. A., & Orr, H. A. (2004). Speciation. Sunderland, MA: Sinauer.
Orr, H. A. (1997). Haldane's rule. Annual Review of Ecology and Systematics, 28, 195–218.
Turelli, M., & Orr, H. A. (1995). The dominance theory of Haldane's rule. Genetics, 140(1), 389–402.
Turelli, M., & Orr, H. A. (2000). Dominance, epistasis and the genetics of postzygotic isolation. Genetics, 154(4), 1663–1679.
Wu, C.-I., & Davis, A. W. (1993). Evolution of postmating reproductive isolation: The composite nature of Haldane's rule and its genetic bases. American Naturalist, 142(2), 187–212.
Laurie, C. C. (1997). The weaker sex is heterogametic: 75 years of Haldane's rule. Genetics, 147(3), 937–951.
Presgraves, D. C. (2008). Sex chromosomes and speciation in Drosophila. Trends in Genetics, 24(7), 336–343.
Schilthuizen, M., Giesbers, M. C. W. G., & Beukeboom, L. W. (2011). Haldane's rule in the 21st century. Heredity, 107(2), 95–102.
Delph, L. F., & Demuth, J. P. (2016). Haldane's rule: Genetic bases and their empirical support. Journal of Heredity, 107(5), 383–391.
Tao, Y., & Hartl, D. L. (2003). Genetic dissection of hybrid incompatibilities between Drosophila simulans and D. mauritiana. III. Heterogeneous accumulation of hybrid incompatibilities, degree of dominance, and implications for Haldane's rule. Evolution, 57(11), 2580–2598.
Price, T. D., & Bouvier, M. M. (2002). The evolution of F1 postzygotic incompatibilities in birds. Evolution, 56(10), 2083–2089.

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This article was last updated on October 27, 2026. It is based on widely recognized original works and textbooks; when citing it in a paper, please check the original sources listed in the references.