Inbreeding depression cause and effect

“Inbreeding depression” is a euphemism for a general lack of health in an individual purebred cat which has been selectively bred (using artificial selection) for appearance, which automatically incorporates inbreeding.

Cat breeders often have to inbreed their purebred cats in order to fix their appearance. This is because the appearance is held on a recessive gene. That recessive gene can only show its effect on appearance if cats are inbred such that they are homozygous for the gene. This means that both copies of the gene are present in the created cat.

Unless both copies are present the appearance is not shown and does not become fixed. So breeders have to inbreed their purebred cats which means offspring mating with mothers et cetera. It’s the kind of breeding that is forbidden for humans under the law in countries such as the UK in order to maintain health. There are different laws for animals.

Inbreeding depression
Inbreeding depression. Illustration by MikeB.
Two useful tags. Click either to see the articles: Toxic to cats | Dangers to cats

Some detail

The best known book on cat genetics, “Robinson’s Genetics” describes inbreeding depression as: “Inbreeding can bring about a decline in vigor or general weakness. This is usually called ‘inbreeding depression'”.

The authors of this book say that inbreeding causes a general weakness in cats and a decline in their vigour. Pretty vague terminology. By ‘general weakness’ I have to presume that means ill health and a compromised immune systems which allows ill-health to come into the cat. And by a decline in their vigour we have to presume that means the cats are less vigorous, less active, more lethargic, dull and uninterested.

My mind actually immediately turns to a couple of blue British Shorthair cats that my late mother bought from a breeder. She gave them away to my late sister and I can see why. I remember seeing these cats and they looked (without being insulting to the cats) awfully stupid. It seems that inbreeding can also compromise a cat’s intelligence.

And the homozygosity of these recessive genes applies to polygenes, meaning more than one gene because the effect of one gene which has a detrimental effect on the cat is not going to be substantial. It is about the cumulative effect mounting up and eventually becoming noticeable in a cat’s health and well-being.

In the general domestic cat population which means moggies or random bread cats, these heterozygotes which are recessive genes that are mutated and have a negative impact upon the cat, are not normally exposed. You don’t know that they are there because they are in a homozygous state. Random bred cats by definition breed randomly. It’s when you artificially select that you get homozygosity.

Robinson’s Genetics also states that “This is not to suggest that all cats carry the genes, or that inbreeding must inevitably produce weak stock.”

They state that the inbreeding of inherently healthy stock is normally quite safe. But some individual cats carry deleterious genes. It is these genes which “make their presence felt due to inbreeding”.

Cat breeders need to watch out for this. And they do watch out for it but regrettably they don’t do enough to tackle it. That is why you see a lot of inherited diseases in purebred cats which shortens the lifespan in general.

Almost any feature of the “normal cat” may be affected according to Robinson’s as follows:

  • A decline in birth weight or vigour. The kittens may be small, thin and/or lethargic.
  • Developmental problems may be present such as poor growth in later life and the adults may be below standard.
  • The litter size may be below average with an increase in the number of stillborn or abnormal kittens.
  • Reproductive performance may be negatively impacted. The male may be reluctant to copulate. The female may be reluctant to come into heat. There may be partial sterility in either sex.
  • At any stage of development a cat suffering from inbreeding depression may have a greater susceptibility to illness such as “the regular appearance of cancer in younger animals” (Robinson’s).
  • There may be “significant losses of cats or kittens to the same disease due to the loss of immunological diversity”. In short, the immune system in a family of cats may be compromised.
  • You may see physical, anatomical defects such as asymmetry, crooked noses and misaligned jaws. The eyes may be uneven in size and be misaligned.

One or several of these problems may be present. The may be small or large deteriorations in the inbred cat. Some cats may be unaffected.

How to counter inbreeding depression

According to Robinson’s it can be countered by “breeding only from the most healthy cats in the programme. This should be true at all times.”

The “cure” is to outcross to say a random bred cat, by which I mean unrelated stock. This will be something that breeders will hate to do as they spend a lot of time and effort to create a certain appearance. Out crossing will erode that appearance.

Alternatively, Robinson’s state that a breeder may be able to “intercross two inbred strains which are displaying early signs of inbreeding depression”. This may produce hybrid vigour also called ‘heterosis’.

It doesn’t always occur but occurs enough that breeders may try it. Purebred cat breeds described as “delicate, temperamental or hard to breed” are considered to suffer from inbreeding depression. It may take years to reach this state of affairs.

It is a condition that can be “particularly insidious and pervade many individuals of a particular breed”. All the breeder can do is to bred from the most healthy cats to prevent a worsening of a poor situation.

The chances of anomalies in the appearance of a cat showing up because of inbreeding are increased with “close inbreeding”.

Robinson’s state that “many recessive defects can be removed and several methods are described in this book”. They even go as far to say that inbreeding can be “a cleansing process” because it brings to light the presence of hidden recessive genes that wouldn’t be seen otherwise than through inbreeding. You can then eradicate them through modifying the breeding process. The point, though, is that do cat breeders do this?

Instead of out crossing as mentioned above, one solution a breeder may adopt to minimise inbreeding depression “would be to increase the size of the stud so that larger numbers are available from which to select the most vigorous animals”.

If you are interested in cat genetics and breeding, I would recommend the book that I’ve mentioned.

Below are some more articles on selective breeding:

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Useful tag. Click to see the articles: Cat behavior

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