Silence of the Xs

Does junk DNA help women muffle one X chromosome?

4:42pm, April 8, 2003

Here's 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 they're 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.

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