The fact that some breeds are larger in population than others and also the fact that the gene pool can be too limited in a small breed is probably obvious to most breeders. But far too many breeders feel secure in that the larger breeds - for instance Persians and Birmans - have a large enough gene pool. No risk of inbreeding problems there, unless you choose to deliberately inbreed a line! But that is not always true.
We sometimes discuss the fact that overbreeding from a few outstanding individuals is harmful for breeds, even the larger breeds. The most extreme overbreeding from one individual that we could imagine is if one single male is mating every female in a population in a generation. With that kind of breeding, of course the gene pool will not be large, even if the population contained a thousand unrelated females. In order to get a better idea about just HOW large the gene pool really is in such a case, we could calculate the effective population. If the population contains 100 individuals, with an equal number of males and females, all of which are mated randomly to each other with the same amount of offsprings resulting from every parent, then the EFFECTIVE population is also 100.
On the other hand, with the extreme overbreeding of one male described above, we can calculate the effective population with the following formula:
1 1 1 ---- = ------ + ------ Ne 4 x Nm 4 x Nf
Where Ne = the effective population, Nm = number of males, Nf = number of females.
Generally speaking, the effective population will not be larger than 4 times the number of individuals of the least represented sex. (Unless you have set up a breeding program specificly designed to avoid losing genetic variation, but that is very seldom the case in cat breeding.) This means that if 5 males are used, the effective population will not be more than 4 x 5 = 20, even if we could use a million different females in that breeding program.
In reality of course it is seldom the case that five males are used for equally many litters and the remaining males in the population are not used for breeding AT ALL. Then it gets a bit difficult to use this formula. But don't worry! There are other methods!
A too small effective population will cause the degree of inbreeding to increase for every generation. There is a connection between inbreeding and effective population. Using this connection we will be able to calculate the effective population of our cat breeds. We can use our pedigrees to calculate the coefficient of inbreeding, COI. The easiest way of doing this is to enter the pedigree in a good pedigree program that is capable of calculating the COI. It is also possible to calculate it by hand. It is not difficult at all, but if the relationships are complicated and you want to calculate the COI from many generations, it is very time consuming, and there is a considerable risk to make a small error somewhere along the way. However, if the relationships are simple and the number of generations to calculate from is reasonable, you could quickly calculate the COI directly from the pedigree.