The coloration of tortoiseshell and calico cats is a visible manifestation of X-inactivation.
In mammals, when a female inherits two X chromosomes, one of the X chromosomes is inactivated so that it will not produce gene products. If it were allowed to produce gene products, there would be twice as many products in females as are present in males, who only inherit one X chromosome. The silencing of one X chromosome is called X-inactivation. When a female embryo is developing, one X chromosome is randomly chosen to be inactivated. This is accomplished by condensing the chosen chromosome into heterochromatin. This condensed form of inactivated X-chromosome is called a Barr body and is easily recognizable in future generations of the cell line, with its inactivation being faithfully copied in subsequent cell generations (Alberts, et al., 473). Specifically, it seems that a noncoding RNA (one that is not translated into protein) produces X-inactivation by coating the entire inactivated X-chromosome and driving the formation of heterochromatin (Alberts, et al., 474). Inactivated X chromosomes also have a high level of DNA methylation. DNA methylation and heterochromatin both are part of epigenetic inheritance, therefore X-chromosome inactivation seems to be controlled by the epigenetic portion of the cell (Bernstein, 671).
- Alberts, Bruce, Alexander Johnson, Julian Lewis, Martin Raff, Keith Roberts, and Peter Walter. Molecular biology of the cell. New York: Garland Science, 2008. Print.
- Bernstein, Bradley E., Alexander Meissner, and Eric S. Lander. “The mammalian epigenome.” Cell 128 (2007): 669-681. Web. Mar 8 2010.