Originally Posted by LilleKat
I'm sure we all know that all tri colours are female....
There are actually 3 ways a male cat can be tri colour or tortoishell.... and they are not all infertile.
1) XXY Syndrome - where a male cat has a genetic anomaly and has an extra X chromosome - normal males are XY. If one X has the orange gene and the other has the non-orange gene, then the male cat will show both black and red in its coat. They are all infertile without exception - there is absolutely no way that any individual with an odd number of chromosomes can produce fertile sperm or ova.
2) Chimerism - where 2 foetuses fuse in the uterus at an early stage of development. If a male and a femal foetus fuse, the resulting cat is XXXY genetically, and can be tortoishell, provided it has both orange and non-orange genes on its combination of X chromosomes. These males have an even number of chromosomes, and are usually fertile. They can produce either black or red offspring.
3) Somatic Mosaicism - best described as birthmarks. The male cat is genetically either red or black, is fertile, but can only pass on either the orange or the non-orange gene. The appearance would be patches of red and black (as in a calico) rather than brindled. This is common in some Rex lines, and really it's the exact same thing as a human having a port wine stain birthmark.
And some red cat facts:
Although in the cat fancy the colour is known as red, the gene that causes it is referred to by geneticists as the orange gene, causing untold confusion to cat owners about the correct terminology!
Contrary to popular belief, cat colour is NOT sex linked - the only sex linked feature of cat colouration is whether the cat produces red pigment instead of black/brown pigment. A red cat still has black/brown genetics, but cannot produce the correct pigment to express those colours. A cat which has black base genetics and the orange gene is usually a very slightly darker shade of red than a red cat whos base genetics are for a brown/chocolate coat - however this is rarely noticeable to the naked eye.