Cat Breeding Genetics

Cat Breeding Genetics

It is certainly true to say that the breeding of cats is now as much a science as it is an art. It will be clear from the detailed descriptions of the breeds available and the varieties within each breed, that cat breeding over the years has attracted an extremely competent range of professionals. The study of genetics or hereditary factors is a science in itself. From the days when Mendel first demonstrated that particular characteristics of the male and female parents could be intermingled, interchanged and studied, the modern geneticist has been able to perpetuate or eliminate characteristics in plants and animals in a highly sophisticated manner. Dairy cows are bred to produce quantities of milk unheard of in years gone by. Varieties of cereals can be developed which are resistant to certain virus or fungal diseases permitting their use in areas of the world previously unsuitable for them. The agricultural geneticist therefore has done much to improve the world’s capacity to feed itself and for countries to grow and produce food previously imported from more favoured agricultural areas of the world.

Although the application of genetic science to the breeding of cats could in no way be considered essential to the survival of mankind, it is nevertheless a fascinating hobby attracting the attention of many learned scientists around the world. Inside the basic cell of every living thing there is a nucleus and although it is perhaps an oversimplification, it can be said that this nucleus contains the ‘secret of life’. Within the nucleus are identifiable strands with what appear under the high magnification of a powerful microscope to be bumps or dots, similar in many ways to beads on a necklace or rosary. The strands are known as chromosomes and the bumps or dots as genes. It is these genes which determine individual hereditary characteristics, which in the cat may be the ones which decide if it is to be a ginger torn or a beautiful pale Chinchilla with dark bands or ticking on each individual hair. Every anatomical feature, from body shape to eye colour to coat type, is registered as a gene on the individual animal’s chromosomes.

Some of the genes are dominant in that their presence will ensure that, for instance, the body shape or coat colour characteristic which they represent will dominate a different coat colour or other feature carried by a less dominant or recessive gene. Thus a ginger-haired cat may carry the gene of a less dominant coat colour which may emerge in a later generation bred from the original ginger animal.

Every cell in a cat’s body will carry exactly the same chromosomes and genes, with one important and logical exception. When mating occurs, the sperm from the male cat fuses with the ovum of the female cat to form a single cell. This then subdivides until a recognizable foetus begins to form in the uterus. Clearly, if all the chromosomes from the male cat joined with all the chromosomes from the female cat the offspring would have twice as many chromosomes as their parents. Nature, however, has taken care of this by arranging that the system by which sperms and ova are formed incorporates a process which splits the chromosomes and therefore halves the number of genes in the individual sperm or ovum. Thus when these join after mating the normal number of chromosomes is present in the developing foetus and subsequently in the adult animal. It is this division and the variations and permutations of it that determines whether the offspring will carry the dominant or recessive gene and therefore show the same coat colour or other characteristic seen in the parent.

Nowadays, there is much talk of genetic engineering which, when applied to certain animals or plants experimentally, can modify the natural process and, when carried to its logical conclusion, enable the experimental scientist working in this field to produce predictable offspring, or types, in progeny from original germ plasm, or more simply reproductive tissue which he has modified. In certain ways, particularly through the effects of radiation, it is possible to modify the individual genes. This sometimes occurs naturally with the unexpected result that a mutation occurs: the offspring produced have an appearance totally different from the characteristics demonstrated by the parents.

A good practical example of this phenomenon in the cat world is the quite unexpected appearance of three oddities: the Cornish Rex, the Devon Rex and the Scottish Fold cats. The Cornish Rex appeared in a litter of kittens from two ordinary short-coated cats which mated in Cornwall, England, in 1950. Ten years later, in the adjoining county of Devon, a similar mutation was observed. The 1950 offspring were named the Cornish Rex and the i960 the Devon Rex. It was naturally assumed, because the unusual coat type in both these breeds seemed so similar, that the genetic mutation was similar in each breed. Cross mating between the two, however, demonstrated that they were in fact separate. The characteristic was perpetuated by breeding offspring back to their mother until cross-mated male and female offspring produced a purebred line perpetuating the type predictably.

Similar mutations can be seen in plants and other animals, for example, blackbirds with white feathering in their plumage. Other than a general reference to the increased radiation risk following nuclear explosions, no satisfactory scientific explanation for such mutations has been offered. It is therefore reasonable to assume that even with the enormous range of types already available in the cat world, an unexpected mutation such as occurred in Cornwall in 1950 could be observed elsewhere in the world, introducing a totally unexpected type of cat to be developed and perpetuated by breeders.

The problem with scientific breeding to perpetuate good points of the breed, by breeding back into the line of offspring to parent or grandparent, is that nature is not always as helpful as she might be. The chromosome carrying a particular genetic factor that is considered desirable may also have a gene with an influence on the offspring that the breeder would prefer to eliminate. In the cow for instance, concentration on milk yield may have introduced a problem of infertility. Some white cats, such as the Angora, may be born deaf and cats with a good coat colour may have a poor coat texture. It is this uncontrollable and therefore unpredictable linking of genetic factors which makes the job of the breeder more difficult and therefore perhaps more fascinating.

Determination of sex by the influence of the X and Y chromosomes is well known. The male sperm may have the X chromosome or the Y chromosome. The female ovum always has the X chromosome. If the male Y chromosome sperm unites with the female X chromosome ovum, an XY chromosome will be formed resulting in the offspring being male. If the male X chromosome sperm unites with the female X chromosome ovum, then an XX chromosome will be formed and the offspring will be female. Some characteristics, carried by these chromosomes, appear in only one sex. Tortoiseshell cats are almost always female because the characteristic of tortoiseshell coat colour is sex-linked. This means that the coat colour characteristic is carried on the X chromosome united with an X chromosome from the other parent. It is possible to produce what appears to be a male tortoiseshcll cat but this is usually sterile and therefore unable to breed with a tortoiseshell female. It may even be an hermaphrodite, an animal which carries anatomical features in its reproductive organs of both male and female, neither of which was sufficiently dominant during the developing, foetal stage to obliterate the other, resulting in an offspring which cannot be said to be either male or female.

This, then, is a very rough and ready guide to some of the mysteries of genetics. Its brevity means that it is necessarily superficial but it does perhaps indicate that the basis of effective breeding is to start with the right gene type and build on that. It also demonstrates that every time a breeder introduces an animal from outside the established breeding line there is a risk that both good and bad points will be introduced. The cat which is introduced produces an ‘out cross’ into the particular breeding line and this is sometimes necessary to improve the type or to reinforce a particular characteristic of the breed which may be weakening in the inbred stock. It is not unusual to introduce a breeding female to stud cats of a totally different type, the purpose being to improve the colour or texture of the coat of the offspring. It may of course take one or two generations before this newly introduced genetic material settles down and produces the predictable results intended.