Genetics Picture

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Red or black pigment

Cats as you know have many different colors, but there are only two different types of pigments: eumelanin, black, and phaeomelanin, red. A male cat can only have one type of pigment, while a female cat can have both (tortoiseshell and tortie). This is because the gene for pigment type is situated on the x-chromosome. The inheritance is thus sex-linked. A male has one x-chromosome and one y-chromosome, and hence, can only have EITHER the allele for black pigment or the allele for red pigment. Females on the other hand have two x-chromosomes and the can have BOTH an allele for black and an allele for red. None of the alleles dominates over the other so the cat will be both red and black - tortie. A female can of course have the allele for black on both x-chromosomes. She will then be black. She can also have the allele for red pigment on both chromosomes, and then she will be red.

Denomination: Xr = x-chromosome with an allele for phaeomelanin, red pigment
              Xb = x-chromosome with an allele for eumelanin, black pigment
              Y = y-chromosome, no gene for pigment

Alternatively: O = red (orange)
               o = black

Remember that a male always must inherit the y-chromosome from his father and thus his x-chromosome will come from his mother. A male cat's pigment will always come from his mother, while his father's pigment is not inherited. A female cat on the other hand gets her x-chromosome from her father. She inherits thus her father's pigment. And she also gets one of her mother's x-chromosomes. She thus inherits pigments from both her mother and her father. Notice that this is only true for genes on the x-chromosomes, so for the colors of the cat it is only true for red versus black. Silver, agouti/non-agouti, dilution and other is inherited regardless of sex.

For example: A tortie female is mated to a black male. The female can either give Xb or Xr to the kittens. The male can give Xb or Y.

___________________________________
|    |               |            |
|    | Xb            | Y          |
|____|_______________|____________|
|    |               |            |
| Xb | XbXb          | XbY        |
|    | Black female  | Black male |
|____|_______________|____________|
|    |               |            |
| Xr | XbXr          | XrY        |
|    | Tortie female | Red male   |
|____|_______________|____________|

The chance of getting a black female is one to four, 25 %. The chance of getting a black male is also one to four, 25%, exactly as the chance in getting a tortie female and a red male.

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