One central reason most researchers think that may be the case is that the Y chromosome is tiny compared with the X chromosome: The X carries about eight hundred genes, while the Y only carries about a hundred to two hundred, leaving large portions of the X un-partnered in the male cell.[*21] The reason that matters, of course, is that in the womb female embryos shut off or “inactivate” one of their two X chromosomes, presumably so they don’t double code for things and gum up the works. Thus, while each cell line carries two X chromosomes in a female-typical body, each living cell is normally
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