Gene Inactivation: The Distribution of Gene Products Among Populations of Cells in Heterozygous Humans

It has long been recognized that in any body cell many genes are inactive; only erythroblasts make hemoglobin, myoglobin is limited to muscle, and no collagen is found in white blood cells. However, until recently it was quite generally assumed that both alleles of a particular gene were either active or inactive. Now an extensive reappraisal of gene action in heterozygous individuals has become necessary. It has been proposed by Lyon (Nature, 1961) and by others (Beutler et al., 1962; Morishima et al., 1962) that at some point in embryo-logical development all of the genes on one X chromosome undergo “inactivation,” and that the progeny of such a cell show the same pattern of “inactivation.” Attempts have also been made to determine whether this concept could be ex-tended to the autosomes.