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My friends' cat recently gave birth however it was later revealed that the father was the cats' son. My friend offered to let me adopt one of the kittens but I am worried that the kitten I adopt will end up having something wrong with it due to the inbreeding.

My question is: How harmful is inbreeding among cats?

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To answer this question, we first need to understand how incest is bad exactly. This has everything to do with genes and chromosomes.

When producing offspring, a mix of the genes of the two parents is combined in the offspring. This is a random process, which is why siblings can be wildly different from eachother.

Normally, both parents are unrelated, so their genes have at least some differences, creating a new combination of genes. When siblings produce offspring, however, their offspring's genes are practically draws from the same genepool.

This means that there is a chance that the offspring gets two identical genes . If both mom and dad got geneA from their father, the offspring could end up with two A genes. This is where problems might occur. Small defects in genes happen all the time, and generally they are caught because of redundancy. Cats have two copies of each gene after all. In most cases, only if both versions are defect, problems arise.

If both genes are identical, we will be certain that these defects will propagate.

However:

The biggest reason animals are biologically programmed to prevent incest is because incest is a evolutionary disadvantage in the long run. Most of the damage done by incest doesn't happen in the first generation. However, continuous incest, generation on generation, will enhance the effect I described above time and time again.

There are many animals, especially those kept and bred as pets, that suffer an enormous amount of inbreeding. A big example are purebred dogs. Without any new genes joining the pool, the entire population of such a breed will eventually start looking incestuous.

But this is on a population level. For a population of animals, incest only becomes a problem after many generations.

Individual

So how does this compare with your kitten?

Let's look at the possibilities: Let's assume your cat's parents have disjoint genes: Mom has AB, dad has CD. Their offspring have four possible sets: AC, AD, BC, BD. The father of your kitten has a 1/4 chance for any of those combinations. Each of those combinations, the father shares one gene with the mom. Either A or B. These is a 1/2 chance for the father to pass on the shared gene, and there is a 1/2 chance for the mom to do so. So in the end, there is a 1/4 chance for a duplicated gene.

That's pretty high, especially since this probability applies to each of the cats genes individually, so the chance for any duplicates is near 1, but the chance for all duplicates is not so high at all.

How harmful is this? Unless there were already genetic defects in the family tree of the parents, I wouldn't worry too much. However, it doesn't hurt to sterilize the kitten, to prevent it from getting offspring of itself. Since that is the population effect starting to weigh in.

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  • sterilization wouldn't really matter. Since the cat is would no longer be living with family members it couldn't engage in more incest. Outside of further incest the genetics of the current kitten are no worse for any future kittens it may produce. Generally I support sterilization due to pet over-population, but I don't think the incest is a factor in that decision. Also, you should really replace the word 'chromosome' with 'gene' throughout your answer, as genetics are recombined on the level of genes, not entire chromosomes. – dsollen May 1 '18 at 15:03
  • @dsollen but even without its family near, it still has a disproportionately high amount of identical genes, compared to other specimens of the same population. It's not as big a deal, I agree, also since mate finding isn't random among pets, but every bit helps. Good point on the genes btw. – JAD May 1 '18 at 16:50
  • @JAD But it only gives half of it's genetic material to the next generation. So it doesn't matter if it has duplicates, it only gives one to the next generation. – user3067860 May 3 '18 at 17:25
  • @user3067860 if you assume the future breeders take care that the mating partner for the kitten is unrelated, and that the offspring doesn't mate further, then yes, on an individual level, it doesn't matter. On a population-genetics level, it still has an impact because it disturbs the Hardy Weinberg equilibrium. Then again, arguably, the effect of a single kitten is like a drop in the ocean. – JAD May 3 '18 at 17:34
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    @JAD No, I stand corrected--I wasn't considering the population level, just the individual level. Also, it turns out that "Hardy Weinberg equilibrium" is exactly the answer to something I was casually thinking about a month ago, which I never got around to looking up, so thank you. – user3067860 May 3 '18 at 17:47
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Inbreeding

Inbreeding can be a problem, but in this case it probably isn't because it's only one time. (that you know of) And non-breed cats have overall less health problems.

Breeders use inbreeding quite a bit to fix traits. But it can have very dangerous side effect for the health of a cat. But genetic defects need to pile up for that to happen.

Do check for health before adopting: does it walk / jump / behave like a kitten that age should? And if it has problems, are you ready to pick up the bill for that? (12 weeks of age for adoption is ideal)


Cats with defects

Even if you have a kitten with a noticeable defect, genetic or not, it can still be a good pet. In that case you will still want a cat that can take care of itself in these ways: litter trained, eating and drinking and simple coat cleaning. I know of one cat that really has some issues, but it can eat, drink and keep itself clean. It has severe movement issues and can't hunt but it still is fun pet to have around.

If you do adopt a cat with genetic defects, please do neuter them.

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