Early in the development of female mammals, one X chromosome in each cell is randomly inactivated to provide an equal expression of X-linked genes in males and females. Through this process, termed X inactivation, many genes on the inactivated X chromosome are permanently silenced and are not transcribed. Once a particular X chromosome is inactivated in a cell, that same X chromosome remains inactivated when the DNA is replicated, and the inactivation mark is passed on to daughter cells through mitosis. This phenomenon is responsible for the patchy distribution of black and orange pigment seen in tortoiseshell cats. X inactivation is a type of epigenetic effect because it results in a stable change in gene expression that is passed on to other cells.
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