Haldane's rule

In humans, barring intersex conditions causing aneuploidy and other unusual states, it is the male that is heterogametic, with XY sex chromosomes.

Haldane's rule is an observation about the early stage of speciation, formulated in 1922 by the British evolutionary biologist J. B. S. Haldane, that states that if — in a species hybrid — only one sex is inviable or sterile, that sex is more likely to be the heterogametic sex. The heterogametic sex is the one with two different sex chromosomes; in therian mammals,^[a] for example, this is the male.^[2]

^ Deakin, J. E.; Graves, J. A. M.; Rens, W. (2012). "The Evolution of Marsupial and Monotreme Chromosomes". Cytogenetic and Genome Research. 137 (2–4): 113–129. doi:10.1159/000339433. hdl:1885/64794. PMID 22777195.
^ Turelli, M.; Orr, H. A. (May 1995). "The dominance theory of Haldane's rule". Genetics. 140 (1): 389–402. doi:10.1093/genetics/140.1.389. PMC 1206564. PMID 7635302.

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[1] Deakin, J. E.; Graves, J. A. M.; Rens, W. (2012). "The Evolution of Marsupial and Monotreme Chromosomes". Cytogenetic and Genome Research. 137 (2–4): 113–129. doi:10.1159/000339433. hdl:1885/64794. PMID 22777195.

[genetics-3] Turelli, M.; Orr, H. A. (May 1995). "The dominance theory of Haldane's rule". Genetics. 140 (1): 389–402. doi:10.1093/genetics/140.1.389. PMC 1206564. PMID 7635302.

[a]

[2]