What is Inbreeding and what isn't?
Inbreeding is the mating of related individuals. In some senses every animal is the result of some level of inbreeding as all members of a species are related, if only very distantly. However, inbreeding is usually defined as matings between fairly closely related individuals. The most extreme inbreeding being that between a father and daughter, or mother and son, followed closely by full sibs mating, then half-sibs, grandparent-grandchild, and first cousins, or uncle-niece and aunt-nephew. In more distant cases there may be only one cat in common on a five generation pedigree.
Many breeders say that they avoid inbreeding, but practice "line-breeding" to "set" type. Don't be fooled, line-bred IS inbred. Cat breeders just give a more acceptable name to the amount of inbreeding that they think is OK. Line-breeding is not a concept recognized by geneticists, inbreeding is and it is all a matter of degree.
I have heard it said that "line-breeding is inbreeding that worked and inbreeding is line-breeding that didn't work". That is a pretty accurate description of the prevailing attitude.
What is the rationale that breeders use for inbreeding?
In the BEGINNINGS of any breed, a certain amount of inbreeding is usually necessary to get the desired breed characteristics. Initially the animals are rare, so it is necessary to breed them together to get more.
However, once the type exists in the breed to the extent that individual members of the breed are easily identifiable by sight as being members of that breed, any real need for continued inbreeding is gone. The genes that cause the type have been already selected for. So long as two animals have the breed type features, you should be able to breed them together and get offspring with type. Of course, as we well know, it is always a game of chance, and any given litter may or may not produce what you personally are looking for.
There is a recurrent myth about line-breeding, that it is "THE" way to get and keep type. It is, in fact, "A" way, not "THE" way. In fact, if you breed two cats together that have good type, chances are better that you will get kittens with good type than if you donít. Some National Winners have come from catteries that avoid line-breeding as much as is possible within their breeds. There are other National Winners from breeders who DO line-breed intensively. The common ground between the two is that both systems rely on cats with good type. (next)
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@Heather E. Lorimer, Ph.D.