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Tortoiseshell cat
Tortoiseshell is a coat colouration in domestic cats named for its similarity to tortoiseshell pattern. Tortoiseshell cats, or torties for short, combine two colours other than white in an asymmetrical distribution, either closely mixed ('brindled') or in larger patches. The two colours always consist of one eumelanistic (black, blue, chocolate, lilac, cinnamon or fawn) and one phaeomelanistic (red or cream) colour. The most common tortoiseshell colouration is black tortoiseshell (black and red). Tortoiseshell can occur in combination with other cat coat patterns, such as tabby and colourpoints. Tortoiseshell cats with the tabby pattern in their eumelanistic colour are tortoiseshell tabby cats, sometimes referred to as torbies or torbie cats.
Like the tricoloured tortoiseshell-and-white or calico (in North American English) cats, tortoiseshell cats are almost exclusively female. Male tortoiseshells are rare and are usually sterile. Tortoiseshell markings appear in many different cat breeds, as well as in non-purebred domestic cats. This pattern is especially preferred in the Japanese Bobtail breed.
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. Tortoiseshell cats are bicoloured and expresses a combination of both next to each other in their coat. The primary gene for cat coat colour colouration (B) produces the brown-toned colours — black, blue, chocolate, lilac, cinnamon, and fawn. This gene can be masked by the co-dominant gene for the orange colour (O), which produces the orange-toned colours — red and cream. The orange gene is located on the X chromosome and has two alleles: orange (XO) and non-orange (Xo), that produce the orange phaeomelanin and black eumelanin pigments, respectively. Typically, the alleles are notated as an uppercase O for orange, or a lowercase o for non-orange.
The (B) and (O) genes can be further modified by a recessive dilute gene (dd) which softens the basic colours. Red becomes cream, black becomes blue, chocolate becomes lilac, and cinnamon becomes fawn. All tortoiseshells form a combination of either two basic colours — red combined with black, chocolate or cinnamon — or two dilute colours — cream combined with blue, lilac or fawn. Therefore, a tortoiseshell cat may be a chocolate tortoiseshell (chocolate and red) or a blue tortoiseshell (blue and cream) or the like, based on the alleles for the (B) and (D) genes. 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". Even though there exist three dilute tortoiseshell colours, blue is sometimes colloquially referred to as "dilute", which is the most common dilute colouration.
Female cats are homogametic (XX) and undergo the phenomenon of X-inactivation, 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. Orange and black females have homozygous O-genes; labelled as XOXO (orange) and XoXo (black).
Male cats, like males of other therian mammals, are heterogametic (XY). 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.
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Tortoiseshell cat
Tortoiseshell is a coat colouration in domestic cats named for its similarity to tortoiseshell pattern. Tortoiseshell cats, or torties for short, combine two colours other than white in an asymmetrical distribution, either closely mixed ('brindled') or in larger patches. The two colours always consist of one eumelanistic (black, blue, chocolate, lilac, cinnamon or fawn) and one phaeomelanistic (red or cream) colour. The most common tortoiseshell colouration is black tortoiseshell (black and red). Tortoiseshell can occur in combination with other cat coat patterns, such as tabby and colourpoints. Tortoiseshell cats with the tabby pattern in their eumelanistic colour are tortoiseshell tabby cats, sometimes referred to as torbies or torbie cats.
Like the tricoloured tortoiseshell-and-white or calico (in North American English) cats, tortoiseshell cats are almost exclusively female. Male tortoiseshells are rare and are usually sterile. Tortoiseshell markings appear in many different cat breeds, as well as in non-purebred domestic cats. This pattern is especially preferred in the Japanese Bobtail breed.
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. Tortoiseshell cats are bicoloured and expresses a combination of both next to each other in their coat. The primary gene for cat coat colour colouration (B) produces the brown-toned colours — black, blue, chocolate, lilac, cinnamon, and fawn. This gene can be masked by the co-dominant gene for the orange colour (O), which produces the orange-toned colours — red and cream. The orange gene is located on the X chromosome and has two alleles: orange (XO) and non-orange (Xo), that produce the orange phaeomelanin and black eumelanin pigments, respectively. Typically, the alleles are notated as an uppercase O for orange, or a lowercase o for non-orange.
The (B) and (O) genes can be further modified by a recessive dilute gene (dd) which softens the basic colours. Red becomes cream, black becomes blue, chocolate becomes lilac, and cinnamon becomes fawn. All tortoiseshells form a combination of either two basic colours — red combined with black, chocolate or cinnamon — or two dilute colours — cream combined with blue, lilac or fawn. Therefore, a tortoiseshell cat may be a chocolate tortoiseshell (chocolate and red) or a blue tortoiseshell (blue and cream) or the like, based on the alleles for the (B) and (D) genes. 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". Even though there exist three dilute tortoiseshell colours, blue is sometimes colloquially referred to as "dilute", which is the most common dilute colouration.
Female cats are homogametic (XX) and undergo the phenomenon of X-inactivation, 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. Orange and black females have homozygous O-genes; labelled as XOXO (orange) and XoXo (black).
Male cats, like males of other therian mammals, are heterogametic (XY). 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.