Question

Tortoise shell cats that are identical twins are very unlikely to have the same splotchy mixture of yellow and black hairs. What is the genetic explanation for this?

Answer 1

In tortoise shell cats the coat color or the hair color is determined by a primary gene B (BB) that is masked by a co-dominant gene for orange color (O). The orange color gene is X-linked and has two alleles X^O (for orange color) and X^o (for non-orange color; black color). Further there is also a recessive gene d (dd) that dilutes or softens the colors.

In female cats, one of the X chromosomes (XX) is inactivated by the process of X-inactivation. If the X chromosome that carries the O allele is inactivated it results in the expression of non-orange allele and the color is determined by the B gene allele. If the X chromosome that carries the o allele is inactivated it results in the expression of orange allele. Further the presence of the recessive dilute gene (dd) softens the color. The inactivation of X chromosome is random that occurs in the very early development of tortoise shell cats. Tortoise shell cats that are identical twins are very unlikely to have the same splotchy mixture of yellow and black hairs because of the random X gene inactivation that occurs in their very early development resulting in different coat colors even among identical twins.

In case of male cats, as there is only one X chromosome that is inherited, X inactivation process does not occur in male cats. The coat color of male cats is determined by the genes that are present on their inherited X chromosome.