The domestic cat has 38 chromosomes, and each chromosome contains more than 25,000 genes. Although this permits a vast array of potential combinations, only small numbers are actually concerned with the bodily features that together define a breed or variety. The great majority are simply responsible for the smooth functioning of the many aspects of the cat's physiology.
Heredity is the random combination of countless genes. The smallest combination of genes that can determine an inherited trait is a pair. A pair is called an allele. One gene is inherited from each parent. When they combine to form the allele, the dominant gene is the one that determines the expression of the trait. The other gene is called a recessive.
For example, if a kitten inherits a dominant gene for a black coat and a recessive gene for blue, the coat will be black. Recessive genes can determine a trait too, but only when paired together. A kitten inheriting two recessive genes for a blue coat will be blue. Still other genes are additive, the trait being expressed by the combined effects of two or more.
When both genes in an allele are identical, the cat is said to be homozygous for the physical trait the allele determines When they are different, the cat is heterozygous. The stud and queen are equally responsible for determining the inheritance of their kittens. When both parents have identical homozygous alleles, either dominant or recessive, it does not matter how the genes sort because all the kittens will inherit the same allele, and all will express the same physical traits as their parents.
In essence, this is the strategy behind most planned breeding programs. The relationship between the various breeding individuals is kept rather close to concentrate the desired genes in the breeding stock. The method is called inbreeding. Close inbreeding involves the mating of parent to offspring and brother to sister. Less close is the mating of half-blood relations such as half-brother and sister, grandparent and grandchild, and cousins. Inbreeding is neither good nor bad. It is a process that exposes both good and bad qualities in the stock. If the strain does carry a mutant recessive gene (more likely to be harmful than beneficial), it is more likely to become apparent with inbreeding. This might be a disaster in the short term, but the negative trait's exposure in the long term is in the best interest of the breed.
A number of different breeds and varieties of the domestic cat have evolved through the accidental or spontaneous appearance of mutant genes. Mutant genes occur rarely, perhaps once in one million offspring. Accordingly, it is not surprising that cat breeders have pursued only about 20 such mutations as breeds despite the large number of cats bred each year. When mutant genes do occur, they are passed along like any others and follow the same rules. Most, but not all, are recessive. The most important mutant genes for breeders are those producing distinctive coat colors. There are only 12 of these, but in various combinations they are responsible for a wide range of cat breeds and varieties. The length and texture of the coat is another essential feature of many breeds. There are at least five such coat-determining mutants that have been adopted. A few breeds are based on a bodily characteristic caused by a single mutant gene. Best known is the Manx breed in which a dominant gene causes the tail to be shortened or absent.
Desirable and undesirable traits can be caused by both dominant and recessive genes. The Scottish Fold breed, for example, is based on the expression of a single dominant gene that causes the tip of the ear to bend forward, giving the characteristic look. A completely unrelated dominant gene affecting all breeds is the gene for extra toes (polydactyly). Extra toes, sometimes as many as seven, are usually found on the front feet.
A dominant gene that may be associated with deleterious side effects is the gene for the completely white coat. This gene also predisposes to a form of deafness that affects one or both ears. Although deafness is more common in white cats with blue eyes, it also occurs in white cats with orange ones.
In the typical short-coated cat, the length of the guard hair is about five centimeters. In contrast, the silky abundant coat of a longhaired show specimen may exceed twice that length. This difference is caused by a recessive gene that appears to prolong the period of growth in hair follicles, so the hair reaches a greater length before entering the resting phase. Selective breeding over many generations for this longhaired homozygous recessive coat trait has resulted in the fuller and silkier coat in Persians and other long-coated breeds.
An example of the additive effect of multiple genes is seen in the range of eye colors typical of many cats. The colors orange, yellow, hazel and green are determined by additive genes. In the Siamese and white breeds, however, eye color is determined by single genes. Body structure and conformation is another multiple-gene effect. In various combinations, polygenes influence the rate of bone growth, the development of muscle and deposition of fat.
Breeding cats to a high degree of excellence demands careful attention to detail and great patience. The modern breeder has far more information on cat genetics than in the past and also has the product of many generations of selective breeding on which to base a successful program.
The object of any breeding program is to preserve the essential qualities and distinct attributes of the breed. Accordingly, a thorough understanding of the breed Standard is a basic requirement. Pedigrees are important because they are the means to study bloodlines and learn the relationships between the various individuals. They are of the greatest value when the individual cats are known or actually have been seen. A pedigree only assures that the animals are registered specimens of the breed. It does not testify to the quality of the cats in question.
Championships do indicate merit and do give some indication of quality. However, they are not always completely informative about the overall superiority of the individuals listed. Some championships are won through the accident of less than normal quality in the competition. The opposite is also true; some cats do not win their medals because of lack of exposure. Since it is the genetic potential of ancestors that determines success, they should possess outstanding qualities worthy of being passed on to offspring. The breeder should possess a knowledge of the virtues and faults of all cats in the pedigree for at least three generations. He or she should have the judgement and experience to pick the best kittens and the willingness to eliminate, as breeding stock, all defective or substandard specimens.
Close inbreeding for three to four generations generally leads to fixing of type, after which further improvement becomes more difficult. At this point the vitality of the strain may begin to suffer. There could be increased incidence of reproductive failure or a lessened resistance to disease. Most breeders have found that it is wise to bring in new blood. The use of a tom from a different bloodline may be considered. This produces an outcross litter and 'reshuffles' the genes that have tended to become fixed in a more or less predictable manner through previous inbreeding. An improvement in health and vigor of the resulting kittens may be apparent from the time they are born without the risks of breeding from unknown ancestry. These kittens are then bred back into the original strain. A single outcross can be advantageous - but the continuous use of outcrosses or the mating of animals who are not related with the hope of finding complimentary qualities is rarely rewarding. It merely breaks up a carefully built genetic constitution and nullifies any progress that has been made in improving the strain.
from Cat Owner's Home Veterinary Handbook
by Delbert G. Carlson, D.V.M. & James M. Giffin, M.D.
with special contributions by Liisa D. Carlson, D.M.V.
Copyright © 1995 Delbert G. Carlson, D.V.M. & James M. Giffin, M.D.
Howell Book House, MACMILLAN, ISBN 0-87605-796-2BACK TO:
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