History
In 1959 Susumu Ohno showed that the two X-chromosomes of mammals were different: one appeared like 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.
The Lyon hypothesis became the Lyon Law on July 22, 2011 at the EMBO 50-years of X-inactivation conference in Oxford.
Read more about this topic: X-inactivation
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