Also known as tricolour or tortie and white the calico isn’t a breed rather a coat colour. Calico cats have a white base coat with patches of red and black.
Cats come in four base (solid) colours, black, red, chocolate and cinnamon. Why not white? Because a cat with a coloured and white coat has a gene which switches off the colour in some parts. The gene responsible for this is the piebald or spotting gene (abbreviated to S), which is dominant. So a white cat is genetically a coloured cat, but the S gene has switched off the colour in certain areas.
To understand why calicos are so rare in male cats we need a little understanding of genetics.
The red (actually known as orange and abbreviated to O) gene is only carried on the X chromosome, making it sex-linked. The O gene is dominant (all dominant genes are abbreviated in UPPER CASE, all recessive are lower case). The mother’s eggs only carry an X chromosome, but the male sperm can carry either an X or a Y chromosome. Each cat will have inherited a chromosome from each parent. So it will be XX (X from the mother, X from the father) or XY (X from the mother, Y from the father).
Interestingly, the faulty gene for hemophilia is also sex-linked and found on the X chromosome. So because females have two chromosomes, the ‘good’ one prevents her from developing the condition, however as the male only has one X chromosome, he will develop hemophilia. This is the same for colour blindness in human males too.
As the male only has one X chromosome, if it carries the O gene (which remember, is dominant), then he will be an orange/red male. If the X chromosome carries the o (non-orange) gene then the male will be another colour, usually black.
If however the female kitten inherits both the O and the o (usually black) gene, then because she has two X chromosomes (instead of an X and a Y like the male), both black and red will show making a tortoiseshell cat. If she also inherits the white spotting gene which is responsible for the white coat colour, she will be black, red and have areas of white.
I like to think of the X chromosome as having four sections (or arms), and the shorter and smaller Y only has three. The extra arm the X chromosome contains additional genes including the O gene. Most genes (except for the sex-linked X and Y) come in two pairs. One from the mother and one from the father. Therefore the male cat only inherits one O gene (which is dominant) from the X chromosome from his mother.
The male only needs one copy of the O gene to be a red cat, however, the female needs two copies of the O gene to be ginger. Which is why female ginger cats are seen less often than males.
For the female inheriting the red and black genes from both parents, the following may occur:
O (from mother) O (from father) = OO=Ginger cat
o (from mother) o (from father) = oo=Black cat
O (from mother or father) o (from mother or father) = Oo Tortie cat
In the male cat, orange is dominant over black. So if he inherits one O gene and one o (non-orange) gene, he will be orange. So the following will occur:
O (from mother or father)o (from mother or father) = Oo =Ginger cat
Because females carry two X chromosomes (one from each
parent), a process known as x-inactivation or Lyonization occurs during fetal development. So every cell has one active and one silenced X chromosome, this includes the melanocytes, which are the cells responsible for hair colour. Therefore the information contained in the active X chromosome is used and the information in the silenced X chromosome isn’t. This results in both male and females only have one X (active) chromosome per cell.
As this happens in the female carrying both orange and black, the random nature of inactivation means both red and black can appear in the coat depending on which X chromosome has been inactivated in the cell.
Just to complicate things more, the calico cat is essentially a tortoiseshell cat with the addition of the piebald (spotting) gene. This gene turns parts of the coat white by switching off the colours in some areas.
As already mentioned, calico cats are almost always female. It is very rare for a male to be calico and when he is, he is almost always sterile. But female calicos are as fertile as any other coloured cat.
There are two possible ways for a male to be calico
If he inherits an extra X chromosome and is therefore XXY. This is known as Klinefelter syndrome.
If two recently fertilised eggs (XX and XY) which have fused together. This means there is one cat, but it has two distinct sets of DNA.