Inbreeding Depression

I am studying inbreeding depression. The statement seems to say that inbreeding can cause depression but what it means is that inbreeding can result in a greater occurrence of two harmful recessive genes meeting and becoming active (homozygous condition). The word "depression" in this instance means a depressed or substandard immune system and depressed or substandard body functions as a result of the coming together of these "bad" genes. That's how I see it anyway.

One of the great challenges for cat breeders is how to breed cats that are consistently in line with the breed standard while maintaining a high level of health. Inbreeding improves the chances of getting the appearance right but increases the chances of health problems creeping into the population. Inbreeding is effectively breeding from too small a population of cats. If you breed more widely using a bigger population you "water down" the chance of two nasty recessive genes meeting. It takes two recessives for the effect of the recessive to be shown so if you can keep them apart the harmful effect will never be shown.

It seems that cat breeders get outbreaks of genetically inherited diseases at which point reactive measures are taken as a matter of urgency. This is probably because breeders strive in a competitive market for the best and perhaps sometimes go too far down the path of inbreeding. The negative results of this then gradually start to show. It then it gets around the breeders and they take reactive measures.

It is better, obviously, to always take proactive steps and breed cats within a sufficiently large population. We must not forget the most important thing of all. We cause suffering to cats if we inbreed too much as some cats will have inbreeding depression. It is not just about winning the show prizes and getting a reputation as a good cat breeder. In fact, I would have thought that in the long term the cat breeder winning the prizes may gain a reputation for breeding unhealthy cats. It's back to long and short term objectives and goals. The long term is always better but harder.

One underlying cause of inbreeding depression is possibly the fact that a number (I don't know the percentage) of cat breeders are not totally up to scratch on genetics. This is a very complicated area - the most complicated part of cat breeding but absolutely central to it. I should imagine that few cat breeders are highly competent in the area of genetics in cat breeding and this is understandable.

I believe that there are too many instances of inbreeding depression (it is worse in dog breeding apparently) which indicates a need to raise standards in cat breeders' knowledge and a need to set good practice standards and enforce the standards. Cat associations should set and enforce standards and provide forums and classes to train in the area of cat genetics.

Gene Frequency

This is one term that comes up when talking about cat genetics and is useful when discussing inbreeding depression. I think the term "frequency" is a little misleading. Gene frequency means the percentage of genes of a certain type (say gene "a") in a population of cats of that demonstrate or have the potential to demonstrate that gene.

Taking a population of 50 cats, there will be 100 places on the chromosomes (50 cats x 2 sets of chromosomes per cat) where the genes can be found. If there are 20 "a" genes in this population of 50 cats, the gene frequency will be 20 out of 100 or 20% (20 divided by 100 multiplied by 100, making 20%).

You can work backwards and work out the gene frequency in a cat population that is suffering from a particular disorder. You can also work out the percentage of cats carrying the gene producing the disorder.

If 20 cats in a population of 100 (20%) suffer from a particular genetic disorder, the gene frequency of the recessive gene causing the disorder is the square root of 20%. 20% can be rewritten as 0.20 (point 2) so the calculation is the square root of 0.2. This is 0.45 or 45%. You can work out square roots using Google. Search for "square root of .2" and the answer comes up top of the page!

You can also work out the percentage of cats in this population of 100 that carry the gene (heterozygous carriers). The math (maths) is this:

(.45 x .55) multiplied by 2 = .49 or 49%


Source: Pawpeds - Ulrika Olsson

Inbreeding Depression to too many cat associations