It's because cats have more different genes for colour than any creature could possibly require.
The gene that controls ginger is totally separate from all the other colour genes, and it only has two versions: 'ginger' or 'not ginger'. A cat always has two copies of every gene, so if it ends up with one ginger gene and one not-ginger gene, it ends up both ginger and not-ginger at the same time; or tortoiseshell.
The thing that complicates this is that the ginger gene is, for reasons best known to itself, on one of the sex-linked chromosomes (the X). So female cats, having two X chromosomes, can have both a ginger and a not-ginger version of the gene, making them tortoiseshell. But male cats only have one X chromosome (partnered by a Y chromosome) and so they can only have one version of the gene, and are always either ginger or not-ginger.
(It's possible for a male cat to be tortoiseshell, because sometimes cell division goes wrong and it ends up with three sex chromosomes (XXY), which could give it two versions of the gene. But that's a genetic abnormality that would also have some other effects, such as making it sterile)
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timeasmymeasure
no subject
Date: 2010-12-15 08:03 am (UTC)The gene that controls ginger is totally separate from all the other colour genes, and it only has two versions: 'ginger' or 'not ginger'. A cat always has two copies of every gene, so if it ends up with one ginger gene and one not-ginger gene, it ends up both ginger and not-ginger at the same time; or tortoiseshell.
The thing that complicates this is that the ginger gene is, for reasons best known to itself, on one of the sex-linked chromosomes (the X). So female cats, having two X chromosomes, can have both a ginger and a not-ginger version of the gene, making them tortoiseshell. But male cats only have one X chromosome (partnered by a Y chromosome) and so they can only have one version of the gene, and are always either ginger or not-ginger.
(It's possible for a male cat to be tortoiseshell, because sometimes cell division goes wrong and it ends up with three sex chromosomes (XXY), which could give it two versions of the gene. But that's a genetic abnormality that would also have some other effects, such as making it sterile)
Did that make sense?