In bi-colored tortoiseshell cats, the melanocytes arrive relatively early, and the two cell types become intermingled, producing the characteristic brindled appearance consisting of an intimate mixture of orange and black cells, with occasional small diffuse spots of orange and black.“ ![]() Pigment genes are expressed in melanocytes that migrate to the skin surface later in development. Cells in which the non-orange ( o) allele is inactivated express the orange ( O) allele. Cells in which the chromosome carrying the orange ( O) allele is inactivated express the alternative non-orange ( o) allele, determined by the (B) gene. “ The cells of female cats, which like other mammalian females have two X chromosomes (XX), undergo the phenomenon of X-inactivation, in which one or the other of the X-chromosomes is turned off at random in each cell in very early development. A cat would need to inherit two X chromosomes – 1 with the orange code and 1 with the black code – in order to display the coat.Īnd then, the cells would have to undergo a process called X-inactivation to produce the brindled appearance of orange and black coloring of torties. However it’s a bit more complicated than this since obviously not all female cats have the coat coloring of a calico or tortoiseshell. So if a cat gets the XY chromosomes, it will be male, and if it gets the XX chromosomes, it will be female.īecause the orange and black color code is attached to the X chromosome, calicos and torties need the double X chromosomes in order to have a chance of displaying the classic coat coloring. The sex chromosomes X and Y determine whether a cat will be male or female. The reason for this comes down to genetics. ![]() About 1 in every 3,000 calicos and 1 in every 3,000 torties is born male.
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