XCI

X-inactivation (also called lyonization) is a process by which one of the copies of the X chromosome present in female mammals is inactivated. The inactive X chromosome is silenced by it being packaged in such a way that it has a transcriptionally inactive structure called heterochromatin. As nearly all female mammals have two X chromosomes, X-inactivation prevents them from having twice as many X chromosome gene products as males, who only possess a single copy of the X chromosome (see dosage compensation). The choice of which X chromosome will be inactivated is random in placental mammals such as humans, but once an X chromosome is inactivated it will remain inactive throughout the lifetime of the cell and its descendants in the organism. Unlike the random X-inactivation in placental mammals, inactivation in marsupials applies exclusively to the paternally derived X chromosome.

In 1959 Susumu Ohno showed that the two X-chromosomes of mammals were different: one appeared similar to the autosomes; the other was condensed and heterochromatic. This finding suggested, independently to two groups of investigators, that one of the X-chromosomes underwent inactivation. In 1961, Mary Lyon proposed the random inactivation of one female X chromosome to explain the mottled phenotype of female mice heterozygous for coat color genes. The Lyon hypothesis also accounted for the findings that one copy of the X chromosome in female cells was highly condensed, and that mice with only one copy of the X chromosome developed as infertile females. This suggested to Ernest Beutler, studying heterozygous females for Glucose-6-phosphate dehydrogenase (G6PD) deficiency, that there were two red cell populations of erythrocytes in such heterozygotes: deficient cells and normal cells, depending on whether the inactivated X chromosome contains the normal or defective G6PD allele.

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