Tortoiseshell coats result from an interaction between genetic, developmental, and environmental factors. Coat colours in domestic cats are produced by the interaction of orange-based
phaeomelanin (O) and black-based
eumelanin (B) pigments. Red becomes cream, black becomes blue, chocolate becomes lilac, and cinnamon becomes fawn. However, due to
genetic dominance, the most common tortoiseshell colouration is
black tortoiseshell (black and red). Various terms are colloquially used for specific colours, for example, black is also called "brown", blue is also called "grey", red is also called "orange", "ginger", and "yellow". in which one of the X chromosomes is turned off at random in each cell in very early embryonic development. The inactivated X becomes a
Barr body. Cells in which the chromosome carrying the orange (O) allele is inactivated express the alternative non-orange (o) allele, determined by the (B) gene. Cells in which the non-orange (o) allele is inactivated express the orange (O) allele. Pigment genes are expressed in
melanocytes that migrate to the skin surface later in development. In bicoloured tortoiseshell cats, the melanocytes arrive relatively early, and the two cell types become intermingled; this produces the characteristic brindled appearance consisting of an intimate mixture of orange and black cells, with occasional small diffuse spots of orange and black. Tortoiseshell cats have a combination of orange-based O and black-based o on two XX-chromosomes; labelled as XOXo, indicating O-gene
heterozygosity. The single X chromosome does not undergo X-inactivation, ergo coat colour is determined by which O-gene allele is present. Accordingly, the male cat's coat will be either entirely orange (O; XOY ) or
melanistic black (o; XoY).
Leonard Doncaster was the first to prove
sex linkage of the tortoiseshell coat colouration; i.e. that tortoiseshell is the female heterozygote of orange and black (XOXo) with the corresponding male being orange (XOY). In the course of his studies he discovered that the rare tortoiseshell male is often sterile. Very rarely (approximately 1 in 3,000) a male tortoiseshell is born; these typically have an extra X chromosome (XXY), a condition known in humans as
Klinefelter syndrome, and their cells undergo an X-inactivation process like in females. As in humans, these cats often are sterile because of the imbalance in sex chromosomes. Some male tortoiseshell cats may be
chimaeras, which result from fusion in early development of two (fraternal twin) embryos with different colour genotypes; these torties can pass only one colour to their offspring, not both, according to which of the two original embryos its testes are descended from. Others are
mosaics, in which the XXY condition arises after conception and the cat is a mixture of cells with different numbers of X chromosomes. == Variations ==