Genetics                                                                                             

This subject seems daunting but once you cut through the jargon, it is fairly simple. There is a lot of information about genetics in cats available but until it was explained to me in simple terms and I had examples, I didn’t understand any of it.

I am going to put the genetics into different sections to make it easier (hopefully), as there are the genes that decide the sex, sex – linked genes and more.                                                                                   

Basic formation of genes and how the sex is determined - 

Cats have chromosomes just as humans do but they only have 19 pairs, (humans have 23 pairs). Basically the chromosomes carry the genes and the genes contain the scripts that make up different bits of the animal. A gene can contain one or more scripts, each doing their own thing.

Just one pair of chromosomes determines a cat’s sex. In the pair a female will have two ‘X’ chromosomes, which is referred to as ‘XX’, a male will have one ‘X’ chromosome and one ‘Y’, referred to as ‘XY’.

The names for the cells involved (egg and sperm) are known as germ cells. 

When mating takes place, the sperm carries either an  ‘X’ or ‘Y’ gene and the egg will carry an ‘X’ gene (as the female only has ‘X’). Therefore if the male passes on the ‘X’ gene, together with the females ‘X’ gene, the kitten will be female and if the male passes a ‘Y’ gene, the kitten will be male. It is the sperm that determines the sex.

The germ cells also passes on copies of genes from the parent, which combined with the copied genes from the other parent, form a new script for that animal’s identity. 

Hereditary and Congenital - 

If something is passed from a parent to the offspring through the genes, it is hereditary. If something occurs because of the combination of genes, not one of the parents, it is congenital. In breeding it is useful to know these terms, as both events are possible, i.e. heart defects can be either hereditary or congenital, but if you have used each parent for other matings and only had this problem with this mating, you would know that it was congenital and not do that mating again. This can help you to decide whether to continue breeding with a particular cat or not.

Dominant and Recessive -

In the combining of the parents’ genes some will not be able to work together as such and need to use only one.

This is most obvious when it comes to colours. Cats are either one or the other, not a mixture of the two, in which case one colour will be more dominant than the other and the offspring will be that colour. This changes because cats can carry many colours and patterns, therefore producing a litter of mixed kittens. The recessive gene is not always the loser and can appear aswell. When it comes to dominant and recessive genes there is no guarantee that predictions will be correct, although they may be much of the time.

Mimic and Masked Genes - 

Mimic genes do what the name implies, they are genes that produce an effect similar to another gene but don’t have anything in common.

Masked genes are genes that are present but unseen due to a stronger gene.

Rogue Genes -

These are the unwanted genes that cause faults such as the Siamese squint. Having kittens born with too many toes (polydactyly) is the result of a rogue gene, along with testicles that haven’t descended and split feet.

Undescended testicles makes neutering a lot more difficult and can put the cat at risk.  

If the cat is carrying rogue genes that have a possibility of causing offspring ill health, it should not be used for breeding kittens.

Sex – Linked Genes - 

These genes are sex specific or reliant on sex and colour combinations.

Torties are always female because the genes involved in making the tortie coat are both orange (O) genes, as the ‘O’ gene is carried on the ‘X’ chromosome, a male can’t be tortie because he will only have one ‘X’ chromosome, therefore only able to carry one ‘O’ gene. 

Reds, creams, torties and apricots carry the ‘O’ gene. If a mating is done between two cats, neither of which being the ones mentioned, there isn’t any chance of producing these. The ‘O’ gene must be in at least one of the parents for it to be produced.

Red, cream and apricot can be produced as males from just one ‘O’ gene, but females are only produced if there are two ‘O’ genes, that is, one from each parent. The example below shows this using red, but the theory is the same with reds, creams and apricots;            

Red (o) + Red (o)       =   red females  

Red (o) + Cream (o)       =   red females

Red (o) + Black       =   no red females   

Pedigrees - 

Once you understand the way genetics work, you will be able to predict which colours your cat carries and what colours will be possible from matings, by reading the pedigrees. A lot of pedigrees have inbreeding at the back of them, so if it is possible you should research as far back as you can, just to check that you aren’t going to combine two already inbred (albeit distantly) pedigrees. Computer software is available that gives inbreeding readings, but these only go back six generations, including the cat itself, or trial matings that go back six generations from the expected offspring, which isn’t always helpful.

The basic pattern of carrying colours is the stronger the colour, the more they can carry and vice versa, i.e.

 (1) A seal point Siamese can carry all of the other colours (ignoring the ‘O’ gene), but being the stronger colour means there’s a chance of seal being dominant 

(2) A chocolate point can’t carry seal but can carry the other colours.

This is the dominance of the basic colours, most dominant first:

The colours in Siamese and Orientals work in the same way so you only need to change things mentally, such as seal to being black or chocolate to brown.

The seal gene is quite simple and only requires one to be present to reproduce that colour. Blue and chocolate differ from this, as it requires two carrying parents to reproduce. Lilac requires both blue and chocolate to be present. 

When reading your pedigree, trying to determine which colours may be carried, also remember the probability factor; if there are five solid generations of seal points, the fact that there is one blue at the back, makes it probable that you won’t carry blue. The way to find out (just in case you really want to know) is to mate to blue (although lilac is a dilute of blue, it would probably be masked by the seal in this situation).

Occasionally colours are carried from many generations back but it is best to concentrate on the six behind the cat, in my opinion, which would be counted as seven generations.

Before beginning the breeding process, feel confident in knowing your cat’s pedigree and what colours it may carry. The next thing is to decide which colours you would like to breed. Although you can’t always get what you want, you can make it more likely to happen. 

The mating outcome tables show which colours may be produced in certain matings.