in model organisms commonly used in the laboratory research setting. A summary of these forms of dosage compensation is illustrated below. However, there are also other less common forms of dosage compensation, which are not as widely researched and are sometimes specific to only one species (as observed in certain bird and monotreme species). Random inactivation of one ♀ X One logical way to equalize gene expression amongst males and females that follow a XX/XY sex differentiation scheme would be to decrease or altogether eliminate the expression of one of the X chromosomes in an XX, or female, homogametic individual, such that both males and females then express only one X chromosome. This is the case in many mammalian organisms, including humans and mice. 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. 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. 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