First cousins having children results in offspring with genetic problems due to lack of genetic diversity. This is established in science.
Does this weakness persist if the offspring then have children from a different genetic pool. How many generations does it take to remove the genetic defect completely?
It may result in genetic problems, with a small risk of that happening:
From Wikipedia:
No cite, but I recall seeing that any problems should be gone in a small number of generations.
It is, but the effect is often grossly overestimated. Babies born to parents who are first cousins are far from certain to have genetic problems; their probability of having genetic defects is increased, but it is not increased hugely. There are studies which indicate that the increase in risk is only about the same as that of a pregnancy when the mother is older than 40. Cite.
This makes it sound like you think a genetic defect is introduced. Nothing new is.
The problem is only the increased chance of a recessive gene coming from both parents, and this expressing itself. If the offspring marry outsiders it will be less likely that their children will inherit that gene from both parents.
Recessive genes can be passed down through many generations and not show themselves. Or they can manifest from two parents, neither of whom had any idea they had the gene. In neither case is there a defect that is introduced or can be removed.
I’m interested in where you find that this is ‘established in science’.
Marriage between first cousins has been and is common or even expected in much of the world. Avuncular marriages (uncle-niece or aunt-nephew) are acceptable in some societies. Genetic risks increase, but not by much unless both carry damaging recessive genes, and not by as much as many environmental factors. So go on, kiss your cousin.
The master speaks : What’s wrong with cousins marrying? - The Straight Dope
Also a SD science advisory staff report :
I don’t think anyone has answered this part of the question explicitly; it has a simple, perhaps surprising answer.
When two unrelated people marry each other, their children do not suffer (more than average) from the homozygosity of alleles that define inbreeding, regardless of how inbred the parents are individually. Had the famously inbred Hapsburg King, Carlos II `el Hechizado’, been able to produce viable sperm and mated with an unrelated woman, the children would have been normal!
Its generations of persistent inbreeding which produce genetic problems. One cousin marriage does not produce off spring with significantly higher risk.
Also, all it take is one generation to marry outside the line and it gets reset.
The question has been answered. To summarize: the reason for higher incidence of genetic problems in close relative marriages is the possibility that both parents carry the same recessive gene and they transmit it to a child. Even if that child is homozygous for that gene, if they marry outside the family, they still have no higher probability than average of mating with someone who has that gene.
At the risk of saying “This makes intuitive sense,”… This makes intuitive sense (and I say that only as a springboard, not to diminish the value of the quoted text).
The real “killer” with cousin marriage and even sibling marriage isn’t the one time it happens in cultures where it’s viewed as taboo and therefore avoided as a matter of routine, it’s where there is cultural pressure to enter into (quite possibly arranged) cousin and sibling marriages over and over again. Not only does the probability of recessive genes meeting their mate increase, but then, when that does happen and there is some sort of genetic condition that might otherwise make someone less attractive as a mate (not just a physical deformity, but perhaps adverse mental and psychological tendencies as well), instead of those conditions being pressured out of the gene pool, they are protected to an extent by the continued pressure to marry within a closed blood-line.
Which isn’t to say that I believe any kind of deformities should be “bred out,” only that any system which forces people to marry into a blood-line, and particularly one that forces generations of people to marry within the same blood-line, will tend to introduce artificial selection pressures that may defeat what might otherwise be filtered out naturally, either through those conditions being selected against by would-be mates who have a choice in the matter, or through the already mentioned pair of recessive genes with someone who doesn’t have the recessive gene.
Note that in a “one-off” situation it depends on how you phrase it:
“The increase in the chances of there being a genetic defect are about 1-2%.” Well, okay then.
“The odds of a genetic defect about double.” Yikes.
Just to expand on this, it’s basically the difference between a “percent increase” and a “percent point increase”, and it’s a great tool if you want to make a risk seem alarming or trivial. If out of a thousand random babies of unrelated couples, one has a genetic problem, and out of a thousand random babies of first-cousin couples, two have a genetic problem. So as the risk goes from 0.1% to 0.2%, is it a 0.1 percent point increase, which sounds mild, or is it a doubling, i.e. a 100 percent increase, which sounds alarming?
I’ve long had the notion that Russian nobility suffered a high rate of hemophilia because of inbreeding. Is this true? How much inbreeding was there?
OK, my memory sucks – but many years ago I went on a tour-thingie that focused on an Australian Aborigine group. They had a complex system of naming their children based on the names of the parents. There were a small number of names available. If you had a certain name, you were not allowed to marry someone with a certain name. Since this group was composed of a number of smaller groups that were nomadic and young people were encouraged (or maybe required) to not marry someone within the smaller group, this system prevented people from marrying someone who was might put the offspring at greater risk for genetic issues. Anyone know anything about this group and how it worked?
No, because, as I understand it, hemophilia isn’t (usually) caused by inbreeding. It’s caused by a defect on the X chromosome. Since men have only one X chromosome, inherited through their mother, if she is a carrier, they have a 50-50 chance of inheriting the disease. Who their father is, and how closely he’s related to their mother, isn’t important, since they aren’t getting any X chromosomes from him anyway. So inbreeding would only be an issue in the (much rarer) case of a woman with hemophilia, who would have to inherit a defective X chromosome from both parents. All of the known royalty with hemophilia were male (and most of them weren’t Russian – the original female carrier seems to have been Queen Victoria, who had one son with hemophilia and two daughters who were carriers).
Fretful Porpentine has answered the first component of the question. But there is another component related to inbreeding: these royal families intermarried a lot. So the gene spread thru other royal families quickly. If they had married outside of other royal families there still would have been kids that got it but far fewer of them would have been prominent royals and so the attention would have been far less.
So the inbreeding made it much more noticeable.
Easy enough to survey, I suppose. Among royalty of the 19th and early 20th centuries, how common was it for one’s mother and one’s paternal grandmother to be related by blood? The last prince of Russia, Alexei Nikolaevich, had hemophilia. His mother was Alexandra Feodorovna and his paternal grandmother was Maria Feodorovna. I’m on my phone at the moment, which does not lend itself to sorting through family trees, but it would not surprise me if the two women were cousins to some degree.
I realize belatedly that my oversimplified take on inheritance was way off base because of course Alexei could not have inherited any X-chromosome defects from his paternal grandmother, though his sisters might have. The more correct study would be of his mother’s genetics and what she may have received from her own mother. I see wikipedia has info on hemophilia inheritance through royal families and I’ll just read that.
We can say what we will, but it’s a scientific fact that the larger the gene pool, the healthier the species.
I know White Supremacists who worship “racial purity” don’t want to hear this, but the fact is that, in genetics, variety is truly “the spice of life”. So, genetically speaking, marrying outside one’s race is not an abomination. It is, in fact, genetically beneficial. So, since marrying first cousins has the opposite genetic effect, it is in essence an undesirable thing to do.
Hybrid vigor a.k.a. heterosis is well studied.
Per that article, mixed breed dogs live 1.2 years longer on average than purebreds. And it has been hypothesized that more people marrying outside their group might explain some of the Flynn Effect: the rise in IQ scores over the decades.
OTOH, for some rare or endangered animals zoo breeders try to breed out harmful traits by keeping the bloodlines small. But this requires culling out any animal with such a trait. Something that isn’t going to happen with humans.
