The Pros and Cons of Inbreeding in Cats

Artificial isolation (selective breeding) produces a similar effect. When creating a new breed from an attractive mutation, the gene pool is initially necessarily small with frequent matings between related cats. Some breeds which resulted from spontaneous mutation have been fraught with problems such as spasticity (cerebellar hypoplasia) in Devon Rexes, skeletal problems in Scottish Folds and the effects of a semi-lethal gene in Manxes. Problems such as hip dysplasia and patella luxation are more common in certain breeds and breeding lines than in others, suggesting that past inbreeding has distributed the faulty genes. Selecting suitable outcrosses can reintroduce healthy genes, which might otherwise be lost, without adversely affecting type.

The more that inbreeding is used to get rid of undesirable traits or to fix a desirable trait, the more likely it is that individuals will also inherit the same set of genes for the immune system from both parents, and be born with less vigorous immune systems. The immune system problem is compounded over successive generations as the animals become genetically more uniform (like the cheetah). According to one theory, immunodeficiency may be caused by a simple lack of heterozygosity in the genes that control the immune system. This is why random-bred cats are generally so robust.

Zoos engaged in captive breeding programs are aware of this need to outcross their own stock to animals from other collections. Captive populations are at risk from inbreeding since relatively few mates are available to the animals, hence zoos must borrow animals from each other in order to maintain the genetic diversity of offspring. In sheep, centuries of selective breeding to improve the quality of wool has caused an important trait to be lost. Ancestral sheep could breed more than once per year. Modern sheep breed once per year. Only recently has the importance of the lost gene been realized (i.e. to increase meat yield), but to reintroduce it from primitive sheep would reduce other qualities selectively bred for over centuries.

Most laboratory mice are becoming so highly inbred that they would probably not survive outside of a sterile laboratory due to poor immune systems (they are generally killed before this becomes a problem in the laboratory situation) and some strains become extinct due to reproductive failure. Many are selectively bred to exhibit defects which will kill them.

Inbreeding holds problems for anyone involved in animal husbandry - from canary fanciers to farmers. Early Turkish Vans were reported to be temperamental, a problem apparently rectified by the importation of new stock. Attempts to change the appearance of Burmese cats in America to produce a cat with a rounder head resulted in cats with congenital problems. Siamese cats have become progressively finer-boned as breeders strive to emphasize the foreign look, resulting in frailer cats in some breeding lines.

In the dog world, a number of breeds now exhibit hereditary faults due to the over-use of a particularly "typey" stud which was later found to carry a gene detrimental to health. By the time the problems came to light they had already become widespread as the stud had been extensively used to "improve" the breed. In the past some breeds were crossed with dogs from different breeds in order to improve type, but nowadays the emphasis is on preserving breed purity and avoiding mongrels.

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