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Mary Lyon effect

Mary Lyon effect The inactivation of one of the X-chromosomes in female mammals; its discovery was predicted by the geneticist Mary Lyon, who observed that otherwise females would have considerably more genetic material than males, since in mammals the Y-chromosome is tiny whereas the X-chromosome is very large. In marsupials it is the paternally derived X that is inactivated, but in placentals paternal or maternal are inactivated at random in different cell lines.

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