Lyon hypothesis

Lyon hypothesis
the hypothesis that the Barr body is an inactivated X chromosome and that the number of such bodies is equal to the number of X chromosomes less one ...

Lyon hypothesis
The Lyon hypothesis states that in cells with multiple X chromosomes, all but one is inactivated during mammalian embryogenesis.

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The coloration of tortoiseshell cats is a visible manifestation of X-inactivation. The "black" and "orange" alleles of a fur coloration gene reside on the X chromosome.

Since females have two X chromosomes, the Lyon hypothesis suggests that one or the other X is inactivated in each somatic (non-reproductive) cell during embryonic development.

See also: Hypothesis, Chromosome, Human, X chromosome, Chromosomes