Evidence indicates that inbred strains DO carry a smaller number of recessive genetic defects, but this is more than balanced out by the fact that they are more likely to suffer from the ones that they have. 

    Crossing out can bring in new "bad" alleles, but so long as those animals arenít line-bred those new genetic defects are highly unlikely to be actually revealed. An animal may carry 10 nasty recessive diseases, but not produce any offspring that are affected, because it is being bred to an animal with a completely different set of good and bad genes. An inbred animal may only carry two nasty genetic defects, but produce one or the other genetic disease in nearly half of its offspring, because it is being line-bred.

    Many health problems with hereditary components (such as cancers, susceptabilities to disease, autoimmune problems, and amyloidosis), do not appear to inherit as simple dominants or recessives. Instead, a SUSCEPTABILITY appears to be inherited. In others the genes involved may have variable expression (producing a wide range in the severity of the condition), or may be partially penetrant (some animals have the gene but not the disease.  These susceptabilities can inherit though, leading to increased incidence of cancer, autoimmune problems, and an assortment of organ system failurres in inbred animals. 

The end result is that overall, inbred animals are less "fit" than outbred ones.

As a result, examining the level of inbreeding and genetic diversity helps to determine how endangered an endangered species is. If the genetic diversity in a population is very high, they are likely be more "fit' as a species, an able to recover more easily from environmental changes or a population bottleneck then a species whose genetic diversity is low.

    There are many breeders in all animal fancy groups that are devoted and faithful advocates of "line-breeding" many of these will never consider any scientific evidence to the contrary as true, worthwhile, or relevant to their particular breed or animal type. I have heard plenty of cat fanciers say "but those studies were done in flies, mice, cheetahs, and cows! Those animals aren't Persians!" or something of that type. There have also been many successful breeders who inbreed almost to the level used to produce inbred laboratory animals, and still have produced top winning animals. Indeed, if their management is very very good, and they are very very lucky, they may even avoid a lot of untimely deaths and major, obvious health and developmental defects. There is, after all, always an element of chance involved. But, why take the risk when you don't have to?

    I am always am amazed that most breeders can see very well that letting their cats roam outside is often dangerous for their cats.  Most breeders specify that cats purchased from them NOT be allowed out side, even though many people let their cats roam and some of those cats lead long healthy lives. Yet the fact that only some people get away with linebreeding while others have disasters, and there is PLENTY of scientific literature emphasizing the dangers of inbreeding depression, does not dissuade people from continuing this inherently dangerous practice.

    Inbreeding ALWAYS affects the immune system.  Whether the effect is noticible or not depends on how well the animals are cared for, whether the breeder has non-inbred animals around who have been raised in the same physical environment for comparison, and LUCK.

Inbreeding and the Immune System

 References