Kitten sex by colour

[Yev]

We have adopted 3 kittens recently. 2 are black and white and 1 have black, white and ginger spots. I've been told that only a female can be 3-coloured. Is that true? I seem to remember a friend who had a massive 3-coloured male cat.

[New Forester]

What you describe is what I'd call tortoiseshell -- with parts of the cat ginger, and other parts "normal" (tabby, black or whatever). Almost (but not quite) all tortoiseshells are female. Female mammals have two X chromosomes, and males have only one. In a male cell the X needs to be able to work the whole cell on its own. However, a pair of such active chromosomes would be too much for the female cell. (The other 22 chromosomes are always paired, so are "half-strength"). In each female cell, one X therefore becomes inactive. At an early stage of development one or other shuts down at random, and descendent cell lines inherit the choice. Female mammals therefore end up with patches of body with one or the other of their Xs inactive. In cats the ginger gene is carried on the X chromosome. Male cats only have one X, so they must be wholly or not at all ginger. Females carry two Xs, so if they have ginger on one and normal on the other they will come out in random ginger patches as a tortoiseshell, depending upon which X is shut down where. A very few males (in humans as well as cats) have an abnormal arrangement, with two Xs and one Y. In these the Xs behave as if they are in a female and one shuts down. This leaves each cell with one active X and a Y, and the animal develops as a male. I think there are one or two genetic syndromes in humans where the X patches can be seen. The tortoiseshell cat thing sometimes gets remembered wrong, and I've occasionally been told "only males can be ginger". This is wrong, although they are more common than wholly ginger females, which must of course have two ginger Xs (and so could only come from mating a ginger male with a ginger or tortoiseshell female).

[FTVS]

new forester - what a brilliant answer

[New Forester]

Thanks FTVS -- I does me best...