Abstract
ACCORDING to Lyon's hypothesis1,2, one of the two X-chromosomes in each XX cell of the normal human female and perhaps in all mammalian females is genetically inactivated at a certain stage of embryogenesis. This inactive X chromosome becomes condensed or heteropycnotic and forms the sex chromatin. Inactivation is presumed to occur at random. The heteropycnotic X can thus be either maternal or paternal in origin. Ohno and Cattanach3 have recently provided direct cytological evidence of this in the mouse. In the skin of animals with a variegated phenotype due to a translocation of the wild-type allele of a coat colour gene to the X chromosome (Xt), the wild-type patches of skin were populated by cells containing a condensed inactive chromosome (presumably the normal X). Mutant-type patches from the same animal contained cells in which a visibly larger chromosome, presumably the Xt, was heteropycnotic.
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