Just thought I’d throw in the comment about the most recent notorious case of interbreeding, Mr. Fritzl of Austria - he kept his daughter locked in the cellar, had 7 children. As I recall, 1 died at birth and one began having seizures at age 19 after living in a locked cellar for all her life. Otherwise, the children, especially the two raised upstairs as “abandoned by mother”, were not bad, no obvious genetic abnormalities, considering the circumstances. A father-daughter cross would be equivalent to a brother-sister cross, the child would have about 1/4 of chromosomes matched up.
Er, well I’m not sure where you were starting from.
The probability based on chromosome assortment is (1/2)^46 = 10^-14, are we agreed on that?
I was not double counting the two children, but previously I was not counting the fact that it has to occur in both the sperm and the egg, so previously I was thinking only (1/2)^23, that’s what I meant when I said it has to happen twice.
Something just occurred to me.
During meiosis, 2 cells are made, each containing half the parent’s DNA. These two cells are complimentary.
So, in the case of a pair of fraternal twins, could two complimentary sperm fertilize two complimentary eggs? Is that possible?
If so, it might happen once every hundred million twins, or so.
…without reading the thread! 
See my post #30 and several subsequent comments, incl Darren re polar bodies.
Where are you getting “hundred million” from?
I’m surprised after 44 posts … no one has noted the sister would be effectively a clone of the mother … the brother of the father … was it safe for the parents to breed?
Maybe not …
Not so. Siblings always get half each from mother and father.
The requirement is that two siblings get precisely complementary halves.
I skimmed it. I somehow missedc #30. My bad.
I think I heard somewhere that there are a hundred million sperm in an ejaculation.
What is special about fraternal twins? How is that different genetically than any other pair of siblings?
Ah, right. I got around that by having two just stick together!
But then you’re missing the random chromosome assortment in the 2N>1N reduction at the end of meiosis, there is still a (1/2)^23 factor from that.
There is no difference, I think Derleth just mis-phrased what he was saying.
Yeah, that’s probably the case.
Yes, fraternal twins are just two different eggs fertilized by two different sperm at the same time - genetically no more significant than any other pair of siblings.
It’s identical twins that have exactly the same genome set and should not try to reproduce with each other…
The worst outcome is expected if you go fuck “yourself”, but of course it’s not recommended for non-identical siblings either. In the former, half the defective recessive alleles in the genome are homozygous in the offspring, in the latter one-quarter.
Of course, identical twins would have a very difficult time reproducing with each other, just because they’re the same sex. Though modern technology might someday make it possible.
During my search for information on polar body fertilization, I came across a link to the case of a set of aborted conjoined fetuses that had male and female genitalia. The parents didn’t allow anyone to study them, so there is only speculation on the cause. One of the speculations is that a single embryo partially split, and that one half lost it’s Y chromosome. If they had been separate, and lived, the female would still have been sterile in that situation (Turner Syndrome) but if something that weird can happen, I’m not convinced that there can’t be some weird combination of genetic and developmental mistakes that could–very, very rarely–end up with “identical” twins being opposite sex and both fertile.
I believe that you are supposed to double-spoiler potentially squicky stuff, so:
As I read once, the reason for sex tests in the Olympics, before the Soviets and their amazing potions, was due to a developmental anomaly. If something interrupts the development process at just the right point, an XY fetus could fail to complete the emergence of male genitalia; the resultant child would appear to be female externally, but the vagina would be very short, no uterus, and the undescended testes would be in the position the ovaries should be and not produce male levels of testosterone. They would grow up to all appearances female, except infertile. They would have a tendency to better physical performance with slightly more testosterone than a typical female, hence over-represented in competitive sports.
there was an episode of house that had a case like that but the illness of the episode was the underdeveloped testes were cancerous and going to be removed … which was stated as a well known effect of course the boy/girl was a model and supposedly soo alluring that no one had bothered to do a test at birth and just said "its a girl "
What made the plot even more squickier was the teen was sleeping with who ever could get what he or she wanted or needed (including the father) and was trying to get the fact that it was just a really cute and delicate boy and not the hot girl that everyone thought it was legally suppressed so the modeling could continue
Jared Diamond discussed this problem when talking about sex and gender differences in “The Third Chimpanzee” and said the same thing - the person as adult is very obviously feminine, with extremely feminized features. The only interesting difference was a usually very short vagina, about 2 inches. Presumably, any sex partners when bottoming out would think “that’s because of me”…
This is CAIS complete Androgen Insensitivity syndrome.
There’s also Swyer syndrome, where the entire female organs develop except the ovaries; again, usually diagnosed from delayed puberty.
I suppose that it’s not really weird, the person is in a respects physically female except for a lack of ovaries and possibly other internal female organs.
This is kinda gross, but it made me laugh. Excellent alliteration.