OK, so let's start with genes and inheritance. All animals (including humans) get half their genes from their mum and half from their dad and the combined effect of each pair of genes determines everything about us. In cats, genes decide what colour their fur will be. Now, like humans, cats have two copies of most genes known as alleles (one from dad, one from mum), carried on pairs of chromosomes (one of each pair from dad, one from mum - you get the idea).
But notice how I said 'most genes'. We all have one pair of chromosomes known as the sex chromosomes. It is this pair of chromosomes that decide whether a baby will be a girl or a boy. You always get an X chromosome from your mother but can get an X or a Y from your father. So half of all babies will randomly end up with two X's and the other half will have an X and a Y. XX=girl, XY=boy.
Unlike all the other chromosomes, these pairs are not the same (they have to carry all the instructions that make girls different from boys). A Y chromosome is smaller than an X chromosome and has less genes on it. So while girls have two copies of all the genes found on the X chromosome, boys only have one copy of those genes present on the one copy of the X chromosome.
So back to the cats. Most of the genes involved in deciding what a cat's coat will look like are found on the autosomes (normal chromosomes that have nothing to do with sex). However, one of the colour genes is found on the X chromosome. This gene is the orange fur gene. It can come in two forms, or alleles – orange (O) or not orange (o).
Because the orange gene is found on the X chromosome, a male cat can only have one copy – O or o. So a male cat is either orange or not orange, in which case other genes will decide what colour it is (black, brown, white, grey etc.)
But female cats have two copies of the orange gene. Things get
interesting when they have both an O and an o. During the development
of an embryo, each cell can switch off one of the X chromosomes
so that only one of each pair is used. In cells where the O allele
is active, the fur will be orange. Where the o allele is in charge,
the fur will be another colour. So the cat is a patchy mixture
of both colours. You never get this happening in male cats as
they have only one X chromosome and this is switched on in every
cell.
