<P> The evidence for this mechanism of dosage compensation was discovered prior to scientists' understanding of what its implications were . In 1949, Murray Barr and Ewert Bertram published data describing the presence of "nucleolar satellites, which they observed were present in the mature somatic tissue of different female species . Further characterization of these satellites revealed that they were actually packages of condensed heterochromatin, but a decade would pass before scientists grasped the significance of this specialized DNA . </P> <P> Then, in 1959 Susumu Ohno proved that these satellite - like structures found exclusively in female cells were actually derived from female X chromosomes . He called these structures Barr bodies after one of the investigators who originally documented their existence . Ohno's studies of Barr bodies in female mammals with multiple X chromosomes revealed that such females used Barr bodies to inactivate all but one of their X chromosomes . Thus, Ohno described the "n - 1" rule to predict the number of Barr bodies in a female with n number of X chromosomes in her karyotype . </P> <P> Simultaneously, Mary F. Lyon began investigating manipulations of X-linked traits that had phenotypically visible consequences, particularly in mice, whose fur color is a trait intimately linked to the X chromosome . Building on work done by Ohno and his colleagues, Lyon eventually proved that either the maternal or paternal X chromosome is randomly inactivated in every cell of the female body in the species she was studying, which explained the heterogeneous fur patterns she observed in her mosaic mice . This process is known as X-inactivation, and is sometimes referred to as "lyonization". This discovery can be easily extrapolated to explain the mixed color patterns observed in the coats of tortoiseshell cats . The fur patterns characteristic of tortoiseshell cats are found almost exclusively in females, because only they randomly inactivate one X chromosome in every somatic hair cell . Thus, presuming that hair color determining genes are X-linked, it makes sense that whether the maternal or paternal X chromosome is inactivated in a particular hair cell can result in differential fur color expression . </P> <P> Compounding on Lyon's discoveries, in 1962 Ernest Beutler used female fibroblast cell lineages grown in culture to demonstrate the heritability of lyonization or random X-inactivation . By analyzing the differential expression of two existing, viable alleles for the X-linked enzyme glucose - 6 - phosphate dehydrogenase (G6PD) gene, Beutler observed that the inactivation of the gene was heritable across passaged generations of the cells . </P>

Which gene is responsible for dosage compensation in placental mammals