This is what’s confusing me about both yours and dtilque’s claims … at first, you say women could get the mutation from in-breeding (which I honestly think is a problem) … then you say in-breeding has nothing to do with it … huh?
Please explain to me like I’m eight-years-old why in-breeding isn’t a problem when one (or both) parents have a recessive (and negative impacting) mutation … Please, pay particularly close attention to my claim that not every child will be effected … thus a single breeding pair of humans is enough to re-pollutitate the world (with some luck of course).
The group will thrive … it’s only the less fit individuals who die off … presumably with the defective genes … as in the case of Victoria’s Hemophilia … the males mostly all bled to death before they could reproduce … which makes it a good case study … which is what I claimed …
Women generally are only carriers; they only rarely get the symptoms. And they become carriers simply by inheriting the gene from their mothers (or in very rare cases, such as that princess you cited, their fathers). They usually won’t become symptomatic unless both their father is a hemophilac and their mother is a carrier.[sup]1[/sup] Inbreeding increases the chances of this happening. But since it was very rare before modern medicine that hemophiliac males survived long enough to breed, it’s pretty much a non-issue. (With modern medicine, we have genetic counseling, so it usually doesn’t happen either.)
I was previously unaware of Prince Leopold. The only way he survived childhood is because they could afford to have doctors attending him throughout that period. And even then, they probably had many scares. The same no doubt applied to his grandson. At any rate there was no inbreeding involved in the Victorian hemophilia.
[sup]1[/sup] Some women carriers can become symptomatic under certain conditions because they’re only producing half as much clotting factor as non-carriers. That doesn’t change the above, though.
That ratio of 1:25 was not chosen to maximize reproduction. It is chosen to maximize profit, which is a combination of money from selling meat plus money from selling milk minus the cost of keeping the animals. If males produced milk, that would radically change the numbers.
It seems to me that if you want to maximize genetic diversity, you would want to estimate how many babies each female can have in lifetime and then provide precisely that many males plus as many females as you have room for. You’d also want a rule which says no female is allowed to get pregnant twice from the same male. For example, suppose you have room for 100 humans in the ark. If you estimate that each female can get pregnant and give birth 12 times, then you’d want 12 males and 88 females. Furthermore, you’d want every single one of them to be young adults (about 18-25 years old) and you wouldn’t want any of them to be siblings or cousins.
But if you only had room for 26, I think 12 males and 14 females would be better than 1 male and 25 females because the very next generation would all be half-siblings.
The most noted example are the Queen and Prince Phillip.
She is Victoria’s great-great grandchild but she could not have inherited the defective X gene because her great-grandfather (Edward VII) did not do so.
Prince Phillip is also a great-great grandchild, from Victoria’s daughter Princess Alice. Technically, he could have inherited the gene, since his descent was entirely through the female line: Victoria => Princess Alice => Princess Victoria of Hesse and by Rhine => Princess Alice of Battenburg => Prince Phillip.
However, since he’s lived into his nineties without any sign of the disease, it’s clear he did not get the defective gene.
The defective gene means the cells don’t make the clotting factor in the blood. Some gene is defective, so the sequence of assembling the protein for the clotting factor fails because one step does not work, the gene is incorrect. This defective gene is on an X chromosome. The general thought is that Victoria or her mother (or somewhere in a matrilineal descent) an X chromosome was copied incorrectly and became defective. Most likely Victoria or her mother, since there are no records of further back generations having hemophilia.
Since it’s on the “X”, a woman has two X chromosomes, so if only one is defective, the other one can still do the job and a female can be a carrier but show no symptoms.
A male, on the other hand, gets only one X and from his mother, so if his mother has one defective X (and one good one), it’s 50-50 that the male gets the failed gene and becomes hemophiliac.
A woman could be hemophiliac if she had two defective chromosomes (perhaps a biologist can chime in here - I presume they both have to fail in exactly the same way). However, one of those X’s is her father’s - so if daddy survived to reproduce, he could pass that along to his children - all his sons would be OK, all his daughters would be carriers. (as happened with Victoria’s son Leopold and so his daughter Alice) But if the mother also is a carrier then the daughters have a 50-50 chance of being hemophiliacs not just carriers - they would get both bad X genes. No instance of this occurs in Victoria’s family.
Until recently, many males did not survive to reproduce. Prince Alexis the Tsaryevich, IIRC, was 10 years old, barely survived several bad episodes despite efforts to keep him from causing bruises - and this was the best care in the Empire… Until the issue was moot. However, some of Victoria’s affected male children and grandchildren survived to their 30’s or later.
How would a daughter inherit the same bad gene from both parents? Obviously, if the parents were closely enough related that the mother is a carrier of the same gene the father had - this is the classic risk from inbreeding. However, there’s no evidence that any such crossing of lines happened with Victoria’s family. the further back the familial relationship, the greater the odds that both parents do not have the same gene, especially one that reduces the chances of successfully living to reproduce; more likely it gets weeded out.
So the problem with Victoria’s X chromosome was just bad luck and social prominence, not inbreeding. She spread it to several aristocratic and ruling families because that’s who they married. It would have been the same result with any family. The biggest contributing factor seems to be that she had 9 children, of which only 2 girls were apparently carriers and one boy was hemophiliac. Whereas the one daughter, Beatrice, had 3 children and both boys had hemophilia and the daughter was a carrier. 9 children, 3 children, same number affected. Luck of the draw…
Nowadays, there is the medicine (clot factor) to let hemophiliacs lead normal lives. This means that it is a risk that totally unrelated hemophiliacs and carriers could meet and pass on the full complement of genes and the disease to a daughter, or the disease to a son. However, thanks to our understanding and DNA technology most people nowadays who know they have debilitating hereditary diseases make an effort to not pass them on.
Just to clarify - yes, Liz and Phil are both great-great-grandchildren of Victoria, but neither of their branches has a history of hemophilia. Phillip’s great-grandmother Alice (different Alice) was a carrier, but his grandma her daughter apparently was not. And of course, Elizabeth, descended through 3 sequential sons, obviously does not have the gene.
According to the wiki article on haemophilia in the European royal families:
• the gene was likely a spontaneous mutation in either Victoria or her mother, the Duchess of Kent. Her father, the Duke of Kent, showed no signs of it, nor was there any prior family history of the disease in the Duchess’s family. Victoria having the gene was therefore not an example of in-breeding.
• Victoria’s daughter, Princess Alice (Phillip’s great-grandmother) is believed to have been a carrier, as was her daughter, Princess Alix of Hesse and by Rhine. Princess Alix married Tsar Nicholas II (becoming Tsarina Alexandra). Their son, the Tsarévitch, had haemophilia.
• there are several confirmed examples of descendants of Victoria with haemophilia. They were all males. That is not an example of in-breeding making the disease express itself, because by definition, males only have one X chromosome, inherited from their mother. Rather, it’s an indication that members of the British Royal family were prized matrimonial partners for the other European royal families in the 19th century.
• there is only one possible example of a female descendant of Queen Victoria who may have actually been haemophiliac: Grand Duchess Marina of Russia, daughter of Tsarina Alexander (i.e. Princess Alix). She apparently showed signs of bleeding during a tonsillectomy. However, even if she was haemophiliac, that was not an example of inbreeding because she was only descended from Queen Victoria via her mother, Princess Alix, and her grandmother, Princess Alix. If she had a defective X gene, it had to be the one from her mother, as her father the Tsar was not haemophiliac.
How? A man can only get the defective X gene from his mother. Whether it was in his father’s family is irrelevant. In-breeding is therefore Ireland in the case of a male haemophiliac.
And since the gene is normally recessive, it usually only shows in a woman if she has defective X genes from both. mom and dad.*
But if dad has the defective X chromosome, he isn’t just s carrier; he actually has haemophilia.
As others have posted, before modern medicine it would be rare for a male haemophiliac to survive to breeding age.
So again, it’s difficult to see how in-breeding would make it more common for the disease to manifest itself in a woman.
The suspected case of Grand Duchess Marina shows that in rare cases, a woman may have symptoms just from one X chromosome from her mother, but that is not particularly common.
ETA: I see that md2000 has made the same points quite nicely while I was composing my posts.
Is the answer 2? Two men and two women? Assuming polygamy and they all mate together, that means their children will be a mix of full siblings, half siblings and unrelated. Isn’t that enough genetic diversity?
Half siblings share 25% of their DNA, but in some cultures people marry their nieces or nephews and those cultures survive. Nieces and nephews share 25% of their DNA with aunts and uncles just like half siblings.
I posted a study earlier showing that when you share 12.5% of your DNA, the odds of genetic defects are not much higher than chance. I’m not sure what the odds are for 25% (like half siblings, or an aunt mating with the male son of a full sibling). If the odds by chance are 3-4%, and the odds if you share 12.5% of your DNA are about 6-7%, then I wonder what the odds are if you share 25%. Even if the odds of a child having a major birth defect are 10-20%, that still means 80-90% of children will not carry those defects.
If two half siblings are born, and one of the half siblings mates with the child of the other half sibling, shouldn’t the genetic risk be as low as the risk from first cousins mating?
So is the answer 2? Assuming those 2 couples do not possess a large number of recessive dangerous genes? Even if they do, is that a big deal? Keep in mind we are talking about repopulation of earth, not having 100% of children be healthy.
Even if both parents carry a recessive gene, that only means 1 in 4 children will carry two copies of the recessive gene. If they have 8 kids, and 6 survive into adulthood and 2 die from a recessive genetic disease, then the human race still survives. 2 parents create 6 kids who survive to adulthood, who create 18 grandkids who survive to adulthood (assuming 1/4 of the grandkids die from the recessive trait). Sure 25% of kids will die, but before modern medicine it wasn’t uncommon for 50% of children to die before reaching reproductive age due to malnutrition and infectious disease. Some women have 20 kids over their lifetime, even if 1/4 of them die that still means the earth gets repopulated.
So realistically, is 1 pair enough? Or is it better to have a minimum of 2?
You may find the short story “Survival Ship” by Judith Merrill of interest. It’s about a trans-generational spaceship that deals with sexuality in an interesting way.
One pair is enough, barring detrimental recessives.
Two males and one female is better than one pair. Two females and one male is better than that pair. Two pairs is better than either than those. Ideally, both males would have offspring with both females.
Of course, the more individuals and the more genetic diversity the better. But one pair is potentially sufficient, with good luck and optimal conditions.
I didn’t not say there is no answer. I said, simply and unequivocally, that one pair was enough. Please don’t claim I said something completely different from what I said. And don’t claim I agreed with you when I didn’t.
“One pair is enough, barring detrimental recessives…But one pair is potentially sufficient, with good luck and optimal conditions.”
so one pair *might *not be enough. True?
I said a* very small number*. One pair is a very small number.
I dont see where we disagreed.
You said “one pair, maybe”. I said “A very small number”.
Sounds like a good deal, except for the increased morbidity and mortality rate for both mother and children in human multiple births. We really are evolved to gestate one kid at a time.
Yeah, I stand pretty thoroughly corrected on the hemophilia thing. I hadn’t fully thought through all of the implications of it being sex-linked. I had thought that the objection was just to the notion of inbreeding “causing” genetic diseases in general, hence my statement that it doesn’t cause them but makes them more likely to be expressed, which is true for most genetic diseases.
And given an answer of “one pair is enough, if you’re lucky”, it’s still worth asking “OK, how many do you need if you’re not lucky?”. Of course, there’s no single answer to that, because it depends on how much luck you consider acceptable, but the point is, there’s still room for further discussion.
Nah, that’s the old standard plot. The new standard says the two men would turn out to be gay or perhaps trans. Or gay AND trans. Hilarity ensues. There’s probably a tvtrope about this, but I’m too lazy to hunt it down.