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Silence of the XsX chromosome inactivation Authors John Travis Issue August 5 2000 Does junk DNA help women muffle one X chromosome Heres a riddle that will stump most people What do calico cats--those popular felines with a patchwork of orange-red black and white fur--have in common with women who lack sweat glands on portions of their bodies A clue Calico cats are almost always female The women and cats illustrate the remarkable fact that all female mammals are actually mosaics of cells with two different pedigrees Early in embryonic life when theyre merely balls of cells female mammals silence one of their two X chromosomes within each cell Each of the cells randomly decides which X--the one inherited from the mother or the one from the father--it will inactivate As these embryonic cells replicate their descendants in the adult animal retain the chromosomal choice that the original cells made In the case of a calico cat the felines parents passed on different versions of X chromosome genes related to coat color As for the women with only a partial supply of sweat glands one of their two X chromosomes must have a gene mutation that prevents patches of skin with the mutation from making the glands The reason female mammals bother inactivating an X chromosome is self-preservation Over the course of evolution the mammalian Y chromosome has degenerated so much that it now shares few genes with its more robust counterpart the X chromosome That means that women with their double dose of X chromosomes have a genetic surplus compared with men To ensure that the sexes work with similar doses of X genes which scientists believe is critical for development female mammals evolved the ability to muffle one of their sex chromosomes In contrast male fruit flies double the activity of their single X chromosome Scientists first recognized the phenomenon of X inactivation 4 decades ago Ever since theyve sought to understand
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