Haldane's rule

Haldane's rule relating to hybrids of species and extended to speciation in evolutionary theory is easily stated:
 * When in the offspring of two different animal races one sex is absent, rare, or sterile, that sex is the heterozygous (heterogametic) sex.

It was originally formulated in 1922 by the British evolutionary biologist J. B. S. Haldane. It is sometimes referred to as Haldane's law.

In many organisms, such as mammals or Drosophila flies, males are the heterogametic sex, in that they have XY sex chromosomes, whereas females are homogametic, with XX chromosomes. However, in some other animals (i.e. birds, butterflies) and plants, the reverse is usually true. Haldane's rule has been shown in a number of different hybrid crosses where either the male or the female is the heterogametic sex.

The fact that hybrid sterility and inviability can evolve due to Haldane's rule in such a vast array of different organisms is quite striking. However, the actual explanation of this phenomenon is rather complicated. Many different theories have been advanced to explain the genetic basis of Haldane's rule.


 * The dominance theory: Heterogametic hybrids are affected by all, recessive and dominant, X-linked genes involved in incompatibilities, while homogametic hybrids are only affected by the dominant ones.
 * Faster male theory: Males genes evolve faster due to sexual selection.
 * Meiotic drive: In hybrid populations, selfish genetic elements inactivate sperm cells (i.e: A X-linked drive factor inactivates a Y-bearing sperm and vice versa).
 * Faster X theory: X-linked have a larger effect in reproductive isolation.

The dominance theory is the leading theory explaining Haldane's rule. However, it is not mutually exclusive and many causes might potentially act together and cause hybrid sterility and inviability in the heterogametic sex.

Haldane's rule has a correspondence with the observation that some negative recessive genes are sex-linked and express themselves more often in men than women, such as color blindness or haemophilia.