Recently in a discussion about something completely unrelated, a person made some very incorrect statements about how inbreeding works. Their argument was that it actually benefits small populations, helping to eliminate disease- and disability-causing genes from the gene pool.
Um, no, that's not how genetics works. At least, not in people. See, breeders of all kinds, but especially those who deal with livestock, use some degree of inbreeding to produce the best traits in the largest proportion of animals. They tend to use quite a few females, but very few males. So a half a dozen males - let's say dairy breed bulls - that have historically produced daughters that calve easily and give lots of high-quality milk may be the fathers of hundreds of children (ah, the miracles of sperm production and freezing), and some of those children may be more males that are also excellent breeding stock and are bred to many of their own half-sisters in an attempt to make the grandchildren cows even better producers. This, in combination with a careful management program and the odd infusion of outside stock, is a very successful way to make a highly inbred, highly successful herd of dairy cattle.
But wait! We're not dairy cattle! The reason inbreeding works to remove less fit alleles from a cattle population is because we kill off all the ones that aren't good enough out of the offspring. We, as human beings, go looking for cows who have poor milk yields, diseases, or defects, and we cull them without letting them reproduce, or in conjunction with not breeding their existing offspring (i.e., selling them for veal). This is a very artificial environment with extremely unrealistic gene flow conditions. We certainly can't artificially inseminate women with sperm from a small group of men chosen to be the breeders and kill (or at least sterilize) all the other men, and we definitely can't selectively kill babies with disabilities and the women that gave birth to them!
Lesson One: We Are Not Dairy Cattle. Human populations cannot be assumed to produce the same results from inbreeding as livestock populations, because the mating system and selection pressures are completely different.
This person quote-mined an article on wild bird populations (of all things!) to try to support the argument that inbreeding makes things better. However, the context of the article was that inbreeding is a problem that can be compensated for by birds mating with non-relatives most of the time even if they mate with a half-sibling once in a while. That is true. However, by compensating for inbreeding the researchers mean "offsetting the likelihood of nest failure" in inbred bird pairs. In other words, birds are far less likely to have surviving offspring in the inbreeding year, but the impact of that on their general reproductive success is small, as long as they mate with non-relative birds in other years, producing live, healthy chicks.
Do humans have babies every year? Technically we're capable of it, but most people don't. And even among those that do, very very few (hopefully none) are mating with a different person every year, raising babies for a year and then getting rid of them only to start over again the next year with someone new.
Lesson Two: We Are Not Birds.
Getting away from the tongue-in-cheek a bit here, let's talk a little bit more about why the initial statement of "eliminate disease- and disability-causing genes from the gene pool" is ridiculous. Not only can we not kill (or forcibly sterilize) people that have disabilities or genetic flaws - hello eugenics movement - but doing so would not eliminate bad genes. Not only are a large number of genetic problems recessive - that is, they are only disease-causing in people with two copies of the bad gene, so carriers are totally healthy - but there are also a number of diseases that are cumulative, ones that are due to spontaneous mutations, and ones that are not fully penetrant.
To explain in more detail, cumulative genetic diseases includes many types of autism as well as Huntington's disease. What happens in these cases is that the bad gene in question is bad because it has an excessive number of repeats in the "junk" part of the gene, and this number often increases in each generation. So someone who has Huntington's disease but does not show symptoms until their 50s may have children with the disease that show symptoms in their late 40s and grandchildren with the disease that show symptoms in their early 40s. There can be many generations with no problems, but once the number of repeats passes a certain threshold, each subsequent generation is affected and over time the disease gets worse. But before that threshold is reached, there is no way to know that the family is carrying the possibility of developing that disease. So inbreeding wouldn't make any problems show up - the gene needs time to copy that section more times, and inbreeding won't make that happen any faster.
Diseases caused by spontaneous mutations are not something inbreeding can help "weed out" either. About a third of haemophilia cases are de novo (Latin: newly occurring) mutations where neither parent had a bad gene. Since there is no way to predict when these mutations will happen, they will occur at random and at the same rate regardless of what kind of breeding occurs. Not to mention spontaneous chromosomal disorders such as Down's Syndrome are also impossible to predict and can happen to any couple, related or not.
Many diseases generally labelled "dominant" do not behave like one would expect when that term is used. Normally, dominant genetic diseases mean that if you have the bad gene, you have the condition. But a few strange cases where diseases thought to be dominant seem to skip a generation helped geneticists discover that many of so-called "dominant" genetic diseases are not fully penetrant; that is, there is not a 100% chance that someone who has the bad gene will show any symptoms of the condition it's associated with. So, in some cases, a person with a genetic condition can have children that are totally unaffected, and yet they may go on to have affected children anyway. Again, so much for using inbreeding to draw out and eliminate genetic disorders.
So, we're not dairy cattle and we're not birds. We ethically can't kill off people that don't meet our standards of what is "normal", we can't produce offspring in litters, and we don't get a chance to start over every year. Not to mention that genetic diseases are not simple enough that we can just eliminate them by breeding - often they are phenotypically invisible, occur spontaneously, or build up over time. They are not something you can "weed out", nor, might I add, is it ethical to encourage people to try to produce fatally ill children just so those genes are exposed.
This idea of inbreeding to strengthen a population is a myth. It won't work and it has serious racist and ableist undertones that make me very nervous, not to mention the genetic problems it can cause. The Human Genome Project has taught us that each human being carries, on average, between 7 and 10 recessive genes that can cause fatal conditions when no dominant gene is present. So producing offspring with siblings and cousins brings with it a serious increased risk of those children being fatally ill. Diverse parental genes are also a benefit to children's immune systems due to a mixing of genes for immune proteins that protect us from disease.
With much increased risks, and no benefits, inbreeding is not a solution to any sort of problem, in any situation. Sometimes it is necessary because there are no other options, but in a world where it's not difficult to meet people outside the family, why anyone would advocate choosing to "keep the gene pool pure", for any reason, is totally beyond me.
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