The mammalian X-chromosome contains significantly more genetic information than the Y-chromosome. This gene dosage is controlled for in females through a process called X-inactivation where one of the X-chromosomes is shut down and highly condensed into a Barr body. Inactivation of the X-chromosome occurs in a stochastic manner that results in females being cellular mosaics where a group of cells have inactivated the paternal X-chromosome and other patches of cells have inactivated the maternal X-chromosome. The most striking example of mosaicism is the calico cat. A calico cat (tortoise shell cat) is always a female. One of the genes that encodes coat color in cats resides on the X-chromosome and exist as either orange or black alleles. Due to the stochastic inactivation, the patterning of orange and black fur is a distinctive quality of calicos.
While the genetic information for the the orange or black coat color exists in all cells, they are not equally expressed. This type of heritable trait in spite of the presence of the genetic material (DNA) is called epigenetic to imply that it is “above” (epi) genetics .
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