I know antidotes are not data, but here’s a copy:
I have two first cousins who got married, and their mothers are identical twins. They have three very normal children and seven very normal grandchildren.
Johnny & Edgar Winter were both albinos, despite having no previous albinos in their family tree.
UIAM, there are just three prominent European families noted for inbreeding: the Habsburgs, the Bourbons, and (though seldom worse than simple uncle-niece matings) the Rothschilds. There was also inbreeding on Pitcairn Island, though perhaps not as severe as some of the Habsburg and Bourbon inbreeding.
Both Navajo and Cherokee tribes have a clan system where individuals are not allowed to marry anyone from either their mother’s clan or their father’s clan.
Nor are anecdotes.
But keep in mind that purebreds are the opposite of hybrid vigor. Mates are specifically selected for certain distinctive traits ignoring any other issues. This could result in significant duplication of defective genes. I’m not into dogs or breeds, but I vaguely recall reading that for example, Dalmatians are known for being high-strung and for hip problems, and “features” like a bulldog or pug’s bashed-in nose was selectively bred for. No surprise that totally unrelated dogs, or dogs with extremely different characteristics, will have very different genes and less likely to have problems.
But then it’s rare that a single gene is responsible for many characteristics- so for example, intelligence is thought to be made up a large number of genes. Ditto good health, disease resistance, etc. (as well as for breed characteristics of dogs) So if only one pair of, say, several dozen genes responsible for intelligence are defective, the manifestation may not be that obvious. With closely related parents, offspring will tend to have more than one of the defective genes pair up, particularly as mentioned above if there has been regular inbreeding over generations. So, let’s say, the more pairs are defective and don’t do their job, the less likely the subject is to display intelligence. (or good health, disease resistance, proper development, etc. - whatever else is compromised.)
That multiple genes are responsible for many characteristics is what mislead naturalists for centuries until Mendel. It seemed that offspring were a blend of the parents because when mixing multiple genes for a characteristic, the net result most like was like a blend. It was the lucky ones (like pea flower colour in Mendel’s plants, or hemophilia) that help make the genetic model more obvious.
And in the rare event that a female is born with hemophilia, without modern medical care, she’s highly unlikely to survive long enough to reproduce (much less than her brothers).
Another important point - from what I’ve read, the settlement of Easter Island was from a single convoy of one or two canoes (but, large ones), possibly 100 to 200 moderately related people. These Polynesians came IIRC from the Pitcairn Island group, the result of successive waves of successively less genetically diverse inhabitants as Polynesians spread quickly across the Pacific over a millennium ago. From that small group, population is variously estimated to have peaked at up to 15,000 closely packed people before the big crash. In all that, there’s no hint of serious problems with inbreeding or excessively high infant mortality, physical or mental shortcomings in the general population.
The natural selection of a system without medical care may have helped the winnowing process, but then you’d expect some evidence of high early childhood mortality.
My thought is that the dangers of inbreeding are overblown unless the family is known for problems.
Hmm… quite a few Pacific Island cultures practiced infanticide (a lot of the small islands had to be very careful with population control given their limited resources - overcrowding might well have been a reason many exploration expeditions were launched). If a culture removes infants that are visibly defective from the gene pool in that manner it might cut down on problems generated by inbreeding.
And certainly, there are islands showing evidence of in-breeding. For example, Pingelap has one of the highest rates of achromatopsia in the world, an inherited form of complete colorblindness. A quick google will get returns on various other conditions that are found at high rates on this or that island but are rare elsewhere.
So in isolated populations, such as on islands, inbreeding does occur and does have some effects. However, where the populations have not died out entirely either the inherited problems aren’t severe enough to prevent survival, or sufficient numbers of people are born without such problems manifesting to maintain a population.
And notably, colorblindness is not a trait that would be easily detected in a newborn. Unless you design clever experiments, you’re probably not going to notice it until the child is talking, at which point they’ll probably object to infanticide.
Correct. The traits we do see in those circumstances tend to be ones that aren’t easily detected at birth, and that don’t impede growing to adulthood. People with those traits may be less likely to reproduce themselves as well.
But infanticide of babies with birth defects was rather common in the past, and the further back you go the more common it seems to have been.
Inbreeding may in fact help remove deleterious alleles from a gene pool, if it happens under strong selective pressure. Recessive traits will be much more likely to express at which point they get removed if they are negative. So they drop in numbers in the gene pool.
“Genetic defect” is not a totally simple concept though. A trait may be neutral or even positive if you have a single gene for it, and be deleterious if you have two copies. A gene causing shorter legs may make you a bit squat with one copy and a dwarf with two.
Or the opposite. There is an X-chromosome gene where one copy makes you colorblind, and two gives superhuman colour perception. Its decreasing in frequency of course.
How likely is colour blindness to be noticed without a kindergarten full of crayons? I remember one mother mentioning that she found out her child was blue-green colour blind when he was about 8. I recall some essay about how there were not necessarily different words for different colours - red-heads are “red” because until recently orange was considered a form of red. Homer’s filler phrase was “the wine-dark sea”. In an environment of not many major varieties and colour not that important, it probably did not matter.
Sickle cell anemia is the poster child for this adaption. One gene makes you less susceptible to malaria, two makes you very sick.
Also, many of the Polynesian islands traded a lot, including I imagine, women. Easter island is notable for being so remote, and when the original Pitcairn group died out (they were never very large), beyond the reach of other islanders.
It doesn’t require an isolated island population or inbred royalty to have an excess in some inherited disease. Just a society that usually does not intermarry with outsiders. So even if members of such populations don’t marry first cousins, they’re usually marrying second, third, and fourth cousins. With enough generations of that, it’s no different than marrying closer relatives.
Example: Tay-Sachs Disease is quite common among Ashkenazi Jews and several other populations. Genetic testing is reducing the occurance of the disease, though.
I think there’s some misunderstanding about genetics.
Not necessarily.
If one copy of the gene is slightly disadvantageous it will decline in a harsh environment, but not always in a more benign environment.
If one copy has no discernible effect then those with two copies will be eliminated (or fail to reproduce) but for those with one it does not matter and the frequency will go up, down, or stay the same due to random factors. Sure, those with two copies get removed, but the pressure isn’t on for those with one. That’s how achromotopisa became so common on Pingelap - having just one gene doesn’t affect your vision (and while those with two are less likely to do well in life and have a large family it’s not impossible for them to do so, so they continue to have children even at a reduced rate). There was a population crash in the past that severely restricted the gene pool. At least one of the survivors carried the gene and since the current islanders are all related to all of those handful of survivors (meaning they’re all cousins of one sort or another) the gene became relatively common because of the “founder effect” and not because it conferred an advantage of any sort.
If one copy gives an advantage (like one copy of the sickle-cell gene provides protection against malaria) then the frequency will go UP even if a double-dose is harmful or fatal, because carriers have an advantage.
I suppose the question is then whether or not “superhuman color perception” confers a reproductive advantage. If it does, it will increase in the population. If it doesn’t, it won’t. I’m not sure how “superhuman color perception” could decrease fitness, but it did then it would be gradually less common.
However, I have never heard any evidence that this trait is in fact decreasing, much less that fact being “of course”. Could you provide a cite for that?
It is, in fact, a rare trait, but that’s because it requires not just inheriting two different color perception genes but also that the normal suppression of one out of two X chromosomes in each cell occur in a manner that leads to both types of photopigment being expressed in the retina in proportions that lead to this ability. There’s a certain randomness in that shutdown of X chromosomes - a woman carrying both genes could wind up with either normal color perception, colorblind (more specifically, color-weak), OR with the better than normal color perception. The outcome is not predictable even if you know the person’s genes.
Genetic testing among the Ashenazi is reducing the expression of the disease, but it’s not really eliminating the gene itself because carriers still reproduce. As a result, the incidence of Tay-Sachs has plummeted among the vulnerable Jewish populations but it has not gone down among other populations that have not been as active with identifying carriers and discouraging them from creating children.
Pre-natal testing is also reducing the expression of the disease by, to be frank, aborting children with two copies of the gene. Again, this does not reduce the number of carriers since as far as I know no one is selectively aborting fetuses with one copy of the gene.
That depends on how much fruit is in your diet. Colorblindness would probably come out with a mother telling her kids, “no, no, the ones that look like this are sweet and juicy, but the ones that look like that are still sour”. Which is, of course, why primates have better color vision than many other mammals in the first place: Because we eat a lot of fruit.
Broomstick, selective abortion of homozygotes could decrease the frequency of some genes, by effectively imposing extremely strong selective pressure against the trait. Though you’re right that it wouldn’t make a difference with Ty-Sachs, which already has extremely strong selective pressure: A child born with the trait won’t live to adulthood anyway.
A harsh environment is one with strong selective pressure.
No, not really. If a single copy of a gene is neutral and a double copy leads to elimination of the carrier, then this gene will be out-competed by genes without this disadvantage. Genes where a single copy is negative just get eliminated faster.
Parents who are both carriers will have an extra loss of one-quarter of their children, statistically. Possibly after investing resources in them. This will decrease the frequency of the gene in the gene pool, as the neighbors without this gene over generations will have more surviving children.
This is too simplistic. It depends on how much of an advantage the gene gives in one configuration compared to how much of a disadvantage the other setup inflicts.
The allele in question is located on the X-chromosome and leads to reduced vision with a single copy. So all the male offspring of a functional tetrachromat will have reduced visual acuity. To have offspring that are also functional tetrachromats will require a partner who carries the gene, and so aslo has reduced visual function.
The selection disadvantage imposed on the single-gene carriers outweighs the advantage gained by the double gene carriers.
Sure, here. It tends to drop out of most mammal species because we are so dependent on functioning vision. A bit better vision is good, a bit worse is death.
Need answer fast?
Kids are clever, and green apples may still look different than red to someone with deuteranopia. The standard Ishihara test cards use numbers so it’s difficult to test pre-school children that way. There are some cards using shapes instead of numbers but it’s rare to pick up any signs of colorblindness in young children and have them tested. I suppose the children of color blind parents might preemptively check.
Also, color blindness is not some greatly undesirable trait. Unless our primitive ancestors lived somewhere that had poisonous berries indistinguishable from safe ones there’s not much reason to select out the genes.
Do you have any more info about this? I have mild tritanomaly and my son seems to have color super perception. I doubt tritanomaly is involved in the condition you refer to but I’m still curious.
I was once asked by an anti-abortion protestor “What if they discover the homosexual gene? And Christian couples start abortion their homosexual children? Would you support that?”
When I said “Yes, they should be free to do that if they want to” his jaw literally dropped open, and he was incapable of speech for about two minutes.
What does this have to do with the OP?