PhysOrg Posted December 23, 2019 Posted December 23, 2019 Sex is an ancient and widespread phenomenon, with over 99 percent of eukaryotes (cells with nuclei) partaking in some form of sexual reproduction, at least occasionally. Given the relative ubiquity and presumed importance of sex, it is perhaps surprising that the mechanisms that determine an individual's sex vary so spectacularly across organisms. Mechanisms for sex determination can depend on environmental signals, such as temperature, or can be genetically based, with one sex carrying different alleles, genes, or chromosomes—or even different numbers of each of these—from the other. The most well-studied system for sex determination is the XY system, which can be found in most mammals. In this system, females have two of the same type of sex chromosomes (XX) and males have two different types of sex chromosomes (XY). View the full article
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