But what if the breed is already inbred?
If a breed or a population is already so inbred that clear signs of inbreeding depression have shown up, for instance a high rate of early cancer or infections, what to do then?
If there are unrelated lines in other countries, of course the best solution would be to increase the exchange of cats between those countries. If such unrelated lines are not available, we will have to outcross to another breed or unregistered cats that fit the standard reasonably well. If enough new genes are mixed into the population, the inbreeding problem will be solved.
A not uncommon objection against these kinds of solutions is that we don't know which new harmful recessives might be introduced into our breed through these outcrosses. That is true, we don't know. What we do know though is that most individuals carry some harmful recessive genes. Many breeders also think that it is better to have a more inbred population less different kinds of genetic diseases, in order to more easily keep them under control. Maybe there are even tests available for those deseases. But, as we will see below, it is better to have lower frequencies of several different harmful recessives than to have a higher frequency of one single recessive.
Assume that we have a population A with a gene frequency of 50% for some kind of recessive defect. We will compare it to a population B with gene frequencies of 10% for five different recessive defects. Both populations will then have the same frequency of harmful genes, but population A has defect genes of only one kind (easy to keep under control) while population B has its defect genes divided in five different kinds.
The risk for a kitten in population A to show the genetic disease is then 0.50 x 0.50 = 0.25 = 25%.
The risk for a kitten to show a genetic disease in population B is 5 x (0.10 x 0.10) = 0.05 = 5%.
This shows that we get considerably less defective kittens in a population which has lower frequencies for several different kinds of diseases. The most effective way to keep a breed healthy is not to try to eliminate the harmful recessives, but to get the frequency down to a such a low level that two harmful recessives of the same kind almost never meet.
Some breeders will hesitate to outcross because they are afraid that the type will be gone for ever. Some breeders are of the opinion that inbreeding (linebreeding) is the only way to get excellent and uniform type. It is true that by using inbreeding you can achieve quicker results in this area. The problem is that you risk the long term health of the cats. It is possible to achieve the same result without inbreeding, although it takes more time. Unfortunately inbreeding is a very tempting shortcut for breeders that are interested in showing their cats. But one should keep in mind that most of the genes that are doubled up by inbreeding have absolutely nothing to do with type. For instance, a human has approximately 30 000 genes, and 98,5% of those are identical with those of a chimpanzee! And still, aren't we rather different from a chimpanzee? How large a part of the genes could different between a Siamese and a Persian? Or a Norwegian Forestcat and a Maine Coon? Or between a Burmese of good type and a Burmese look alike moggie with reasonably good type? Not more than what we can fix with some generations of selective breeding, I'm quite sure about that!