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Published online by Cambridge University Press: 01 July 2016
The sex of a zygote is often decided by an XX/XY sex-determination mechanism. One important property of this system is the approximate numerical equality of the two sexes (at least at fertilization) due to mendelian segregation. Does the obvious evolutionary stability of the XX/XY sex-determination system pose a difficult problem to population genetics? We study the fate of new mutations occurring in populations with an XX/XY system, where the mutations are such that they change the sex-determination system. If such mutations occur frequently and the conditions for their initial increase are weak, then the evolutionary stability of the XX/XY system is hard to understand.