Genetics Picture

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Genetic problems are showing up in the breeds!

Yes, that is also due to a too small effective population! Unless it is due to a breeding without regard to the anatomic functions of the animal. Breeding for extremely long bodies might cause problems with the back, and breeding for very short faces might give problems with the teeth, breeding for extremely triangular, square, round, etc. heads might cause problems with jaws, eyes, brain, or whatever. A cat must be allowed to first and foremost be a CAT. It is not a piece of clay that we can shape after our own estetic ideals. A cat doesn't consist of circles, triangles, squares, or other geometric figures, we have to remember that. Maybe we should breed all cats with the coats of a poodle, so that we could CUT out the geometric shapes and odd angles that we find attractive. Then the cats could have their anatomy in peace. No, even if the standard says that a head should be triangular or square, we as breeders must resist to go to extremes. It should be the head of a cat - not a geometric figure.

Except for this breeding for extremes, it is the too small effective populations that cause the high frequencies of many genetic diseases to show up in breeds. Many breeders seem to be a bit confused about this. They might think that if we have for instance 10% of the cats in a breed affected by PRA, meaning a gene frequency of approximately 32% for the recessive PRA gene, and if we don't test and work to reduce this frequency, then the frequency will automaticly increase with time. This is of course not correct. If it were, then also the frequency of diluted (blue, cream, etc.) cats would increase all the time unless we selected against the dilution gene. If the effective population is big enough, and no selection for or against PRA is made, then the gene frequency will stay at 32%.

On the other hand if we select ever so weakly against PRA, for instance let the cats who themselves have PRA (homozygots) have no more than one litter, then the gene frequency will decrease. Slowly with a weak selection, more quickly with a strong selection.

But then what happens if the effective population is too small? What will then happen with the gene frequency? It will be the same effect as if you toss up a coin 10 times. Your chance for heads is 50% every time. And if you had tossed up this coin 1000 times, you would have got quite close to 50% heads and 50% tails. But now you only toss it 10 times. Then it is not very surprising if you by chance got 70% heads and 30% tails, or 30% heads and 70% tails, or something like that.

In the analogous scenario in a small cat population, it means that the gene frequency of around 30% in the next generation might have increased to 35%, because of this random effect. Or else it might have decreased to 25%, because of the same random effect, which in the PRA case would of course be a lot nicer. But let's be pessimistic and assume that the frequency increased to 35%. Then the EXPECTED value of the gene frequency for the next generation is also 35%. But by chance it might end up at 29%, 34%, 38%, 42%, or whatever. The smaller the effective population is the larger the risk is of getting a large deviation from the expected value of the gene frequency. Then this frequency, that we got randomly, will be the expected value for the next generation. This phenomenon is called random drift. If the impact of this random drift gets stronger than the impact of the selection - natural or artificial - then the changes of the gene frequency could very well be the opposite to what we wanted. DESPITE the selection. Then the eyes might get paler in our Siameses, or the lynx tufts might get smaller in our Norwegian Forestcats, or PKD might get more common in our Persians. That would of course be anything but funny!

If we now look into why PKD became a far too common problem in the Persians, it could hardly be caused by some mysterious selection in favour of lumpy kidneys. It must have another cause.

It must of course have started with a mutation in a cat long ago. It was a dominant gene, so the cat developed cycts on its kidneys. Let us assume that it was a male that died from the PKD at 5 years of age. Or perhaps at 7-8 years of age. Either way, we have a certain selection against the gene. If the population is then large enough, the frequency will then decrease and eventually go down to 0%. And even if there were no selection against the gene what so ever, there would be a good chance for the gene to disappear within a few generations, since the frequency could by chance be slightly larger or slightly smaller. And since the frequency was initially very small (one mutated gene in a large population), it is rather likely that the frequency by chance happened to decrease to 0%, and then the gene is gone.

So, the effective population for the Persians apparently was not large enough. Random drift resulted, and by chance this unfortunately caused an increase of the frequency of the PKD gene. In spite of a certain amount of selection against the gene, the result was that the frequency ended up at approximately 25-30% before more breeders became aware of the problem and a stronger selection was introduced.

What does all this tell us? That if we don't have large enough effective populations, then high frequencies of unpleasant genetic problems will continue to pop up. If we are unlucky we might also have difficulties attempting to reduce these problems with selection.

If we instead make sure that we have large enough effective populations in our breeds, genetic diseases will not pop up as a common problem in the entire population. And as a bonus we avoid inbreeding depressions and bad immune systems.

To breed with too small effective populations, and at the same time start projects to fight against genetic diseases within a breed, is just like being treated for lung cancer and continue smoking. Or to scoop out and wipe away the water that have poured over the rim of the bath-tub, while we still haven't remembered to close the tap, so the water is still pouring INTO the tub.

To work for large enough genepools is a kind of preventive medicine measures for the breeds. It doesn't seem clever just to treat the ailments, without bothering about the preventive measures that could prevent the ailments to arise in the first place.

We should also keep this need for reasonably large effective populations in mind when we create and accept one spotted shorthaired cat breed after the other, and one large furry semilonghair breed of medium head-type after the other, etc. Unless the number of breeders willing to work with spotted shorthaired breeds is increased at the same rate as the increase of the number of breeds, the reqruitment of for instance Bengal breeders - just an example - will be at the expense of the Ocicats', Spotted Oriental Shorthairs', Egyptian Maus', etc. possibilities to keep a constant and large enough number of animals for breeding in their breeding programs. Can these breeds afford this? Will the new breed be able to create a place for itself among all these other breeds? Or perhaps they will all get too small populations, so that we in the end have destroyed all the spotted shorthair breeds? These are important things for us in the cat organisations to think about. These threats are real, not just "in theory", and we have already begun to see the first effects, although not yet as badly as in the dog breeds. However, now we have the chance to avoid getting into as bad problems as the dog breeders! We can learn from their mistakes and change into a healthier way of breeding, or we can continue as before and end up in problems.

So this is something that we have to start working with. It must not be forgotten among all the specific projects against specific diseases. It is about the actual basis for the health of the cats and breeds.