There are several genes which control cat coat color, and they interact in a phenomenon called "epistasis".
One of the main determinants for color is the gene for Orange, which has two possible alleles (forms): XO, orange, or Xo, non-orange. XO, orange, is the dominant gene; that is, if the cat has O, then it will be orange, period. And these genes are written with a preceding X because they are on the X chromosome. Keep that bit in mind.
Another gene involved in color is the Black gene, which has three possible alleles: B, black, b, brown, and bl, light brown. Black is dominant, overriding the b and bl forms. However, the two genes, for orange and black, interact: XO suppresses any form of the Black gene, including B. Xo allows the expression of whatever form of Black the cat carries, whether B or b or bl.
Then there is a third gene involved, the "white spotting" gene, S or s. The Spotting gene simply creates white spots -- if the cat is SS, the white spots will be large. If the cat is Ss, the white spots will be small and highly variable. If the cat is ss, any white spots will be miniscule or nonexistant. Pure white cats are different, they have another, different gene, the "white masking" gene, W -- but we're leaving that out of this discussion.
Now, as lahdeefrigginda noted, female cats have two X chromosomes, and males have only one. This IS the key part. Because the O gene is on the X, a male cat will either have O or o. A female cat can have both. Now we get to the cool part -- "X inactivation".
Having two X chromosomes doesn't actually mean that we -- or cats -- _use_ two. In fact, there are some genes on the X chromosome which are damaging if they are overexpressed, that is, if we have two active copies going at once. So what happens is that one of the two X chromosomes gets "turned off"; certain chemical groups get attached to the chromosome so that genes on that chromosome can't be expressed. We say that those genes are suppressed.
However -- here's the key -- it isn't the same X chromosome which gets turned off in every cell. In some cells it will be the X chromosome from mama, and in some cells it will be the X chromosome from papa.
Now, if a cat is an Oo genotype -- that is, if she has the O form of Orange on one chromosome, and the o form of Orange on the other chromosome -- then in some cells, the O form will be active and the o form suppressed, and in other cells, the o form will be active and the O form suppressed.
So to get a tortie, you usually need a cat with two X chromosomes and the following genotypes: Oo-b[B/b/bl]-S[S/s] , where the genes in square brackets can be any one of the listed forms.
In the skin cells where O is active, hair from that cell will be orange, because O overrides the Black gene.
In the skin cells where o is active, hair from that cell will be brown, because o allows the Black gene to express.
And because of Ss, there will be random splashes of white hairs, because the Spotted gene overrides the other two.
And then these are all affected by other genes for pattern, the Dilute gene, the Agouti gene, and the Tabby gene....but let's not get into that....
So how can there be male tortoiseshells at all, given the need for two X chromosomes?
Cats are prone to what we call mosaicism -- this means that they end up with strange numbers of chromosomes in some (but not all) of their cells, because of abnormal chromosome separation during cell division. In most animals, mosaicism often results in various forms of illness. In cats, for whatever reason, it really doesn't seem to bother them much. This means, because cats can stand having different numbers of chromosomes in different cells without being seriously unhealthy, and because it does seem to happen more often in cats than in most mammals, you WILL sometimes end up with male cats which are in fact XY/XXY mosaics. That is, in many of their body's cells they are XY, and thus male; but some of their cells will be XXY, and if these occur in the skin, can create exactly the effect described above.
I realise this is more information than you probably wanted, but I hope it gives you some insight. Cat genetics are fun! (For certain values of "fun", anyway.)
Comments
amazing!
by BROADWAYTHECAT on September 17th, 2009