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Old 12-01-2016, 09:00 AM
Annie-Xmas Annie-Xmas is offline
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Could two siblings share no DNA whatsoever? Is so, could they safely have children?

Since you get half your DNA from each parent, could two siblings have no DNA connection whatsoever? And how little DNA would they have to share to safely have children?
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Old 12-01-2016, 09:03 AM
Boyo Jim Boyo Jim is offline
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This isn't my area of expertise, but I don't think it's possible to get DNA from only one parent.
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Old 12-01-2016, 09:04 AM
Derleth Derleth is offline
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Two siblings could be fraternal twins and safely have children if they didn't have any dangerous recessive traits. In-breeding doesn't create genes, it just shuffles them around, just like sex between two unrelated people; the difference is that in-breeding has a higher chance of making otherwise-unusual combinations.

Last edited by Derleth; 12-01-2016 at 09:06 AM.
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Old 12-01-2016, 09:08 AM
Leo Bloom Leo Bloom is offline
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Too bad we're not like lady rabbits and can carry fetuses from two different male donors at the same time.

a) I just learned that and I thank OP for giving me the opportunity to force it into a conversation.
b) It might not even be true, but somebody just told me that.
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Old 12-01-2016, 09:08 AM
CandidGamera CandidGamera is offline
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I'm going to assume she means that, for instance, the daughter gets all of her DNA from the mother's 'A' side and the father's 'B' side, and the son gets all his from the mother's 'B' side and the father's 'A' side.

Or to simplify and look at one chromosome to illustrate :

Mom has XX. Dad has XY. Daughter has XX. Son has XY. They clearly got different DNA from Dad. So as long as they each got copies of different X Chromosomes from Mom, they are genetically distinct for that pair.
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Old 12-01-2016, 09:09 AM
Annie-Xmas Annie-Xmas is offline
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Fraternal twins can have different fathers, if the mother released two eggs and had sex with two men in a very short period of time.
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Old 12-01-2016, 09:14 AM
Iggy Iggy is online now
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It is theoretically possible that full siblings could not share any DNA, other than mitochondrial DNA. But the odds are so astronomical as to make it impossible.

As eggs and sperm are produced there is a process called crossing over that effectively shuffles the alleles between each of the pairs of homologous chromosomes.

To have two full siblings not share any DNA then all of the crossing over would have to happen at precisely the same locations during the production of eggs or sperm. Then the chromosomes would have to assort in a suitable manner. And the right egg and sperm would have to meet up to create the "opposite" sibling.

Not going to happen. Ever.
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Old 12-01-2016, 09:16 AM
Iggy Iggy is online now
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Originally Posted by Leo Bloom View Post
Too bad we're not like lady rabbits and can carry fetuses from two different male donors at the same time.

a) I just learned that and I thank OP for giving me the opportunity to force it into a conversation.
b) It might not even be true, but somebody just told me that.
It's not true.

It is possible for a woman to carry fraternal twins who are half-siblings genetically. Just takes input (pardon the pun) from two male sources, each fertilizing a different egg.
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Old 12-01-2016, 09:17 AM
chrisk chrisk is offline
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Okay, going back to the original question and assuming you mean full siblings...

Generally, as I understand it, a child receives 1 of each chromosome pair from each parent. Therefore, if we ignore genetics crossovers and such, the chance that two non-identical siblings receive exactly "opposite" chromosomes from both parents would seem to be the same chance that they get exactly the same chromosomes for both parents... 1 chance in 246 (Since there's two chromosomes in each pair, and they're getting 46 chromosomes total from both parents.)

This includes accounting for the fact that one sibling would have to get the X chromosome from the father, and the other the Y chromosome, thus they will be different genders.

That's something like 1 chance in 64 trillion, as I make it.
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Old 12-01-2016, 09:19 AM
Darren Garrison Darren Garrison is offline
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In theory, sure, just like you can in theory flip a perfectly balanced coin as "heads" a million times in a row. But it is very, very, very, very, very, very, very, very, very, very, very, unlikely. If no crossing over took place, the odds would be one chance in 8,388,608 (if I'm doing my math right.) With crossing over, you can add an undetermined number of zeros to that number.
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Old 12-01-2016, 09:34 AM
Leo Bloom Leo Bloom is offline
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Originally Posted by Iggy View Post
It's not true.

It is possible for a woman to carry fraternal twins who are half-siblings genetically. Just takes input (pardon the pun) from two male sources, each fertilizing a different egg.
Is this referring to the White Mom Has One Black One White Twins! one in a zillion case (also in GQ somewhere)?

Before I go look it up, since I started it, I was told this was a constant thing with rabbits, an additional component of their famous fecundity.

Speaking of hermaphroditism, let alone parthogenesis, I myself would be famously fecund could phalanx fascia be fertilized.

Last edited by Leo Bloom; 12-01-2016 at 09:35 AM.
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Old 12-01-2016, 09:34 AM
Anaglyph Anaglyph is offline
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Originally Posted by Darren Garrison View Post
In theory, sure, just like you can in theory flip a perfectly balanced coin as "heads" a million times in a row. But it is very, very, very, very, very, very, very, very, very, very, very, unlikely. If no crossing over took place, the odds would be one chance in 8,388,608 (if I'm doing my math right.) With crossing over, you can add an undetermined number of zeros to that number.
However, you will alway have crossovers, since these keep the homologous chromosome pairs together. Theoretically, if you could isolate the two cells generated in the first meiotic division in both oogenesis and spermatogenesis, you could generate complementary twins. Since this division is very asymmetric for the egg cell, producing a large egg cell and a small pole body, you would probably have to transfer the nucleus of the pole body to a poper egg cell.
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Old 12-01-2016, 09:38 AM
John Mace John Mace is offline
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Originally Posted by Leo Bloom View Post
Is this referring to the White Mom Has One Black One White Twins! one in a zillion case (also in GQ somewhere)?

Before I go look it up, since I started it, I was told this was a constant thing with rabbits, an additional component of their famous fecundity.
It's just that the way you phrased your original statement you made it sound like it was impossible for humans to do it. That's all. It may be super common in rabbits and very rare in humans, but both can do it.
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Old 12-01-2016, 09:39 AM
Peter Morris Peter Morris is offline
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Two things get passed down from parent to child almost unchanged: mitochondria and Y-chromosomes.

Mitochondria are passed from mother to all; her children. There is a minute amount of mutation in each generation, so it changes gradually over centuries, but any two siblings will have mitochondrial DNA as near identical as makes no odds.

Y-chromosomes get passed from father to son. At each generation there is some gene exchange during meosis, and some mutation, but a pair of (non twin) brothers will have Y-chromosomes that are about 99% the same.
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Old 12-01-2016, 09:44 AM
Peter Morris Peter Morris is offline
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Is this referring to the White Mom Has One Black One White Twins! one in a zillion case (also in GQ somewhere)?

Before I go look it up, ...

Here it is
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Old 12-01-2016, 09:47 AM
Leo Bloom Leo Bloom is offline
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Thanks. I owe you one (1) look up.
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Old 12-01-2016, 09:47 AM
Darren Garrison Darren Garrison is offline
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However, you will alway have crossovers, since these keep the homologous chromosome pairs together.
But you could (in theory) have two siblings formed where the exact same crossover locations happened for every chromosome, but each sibling got the opposite set of segments. Probably not enough time between now and the heat-death of the universe to make that likely to happen, but it could.
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Old 12-01-2016, 09:48 AM
Iggy Iggy is online now
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Originally Posted by Leo Bloom View Post
Is this referring to the White Mom Has One Black One White Twins! one in a zillion case (also in GQ somewhere)?

Before I go look it up, since I started it, I was told this was a constant thing with rabbits, an additional component of their famous fecundity.
...
Pretty much as you describe. How common it occurs is not known since in many cases it could go undetected absent a reason to do a DNA test.

But the phenomenon is common enough to have a scientific term, heteropaternal superfecundation. Wikipedia references a study involving paternity suits in which 2.4% of fraternal twins tested were found to have different fathers.
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Old 12-01-2016, 09:53 AM
Leo Bloom Leo Bloom is offline
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Oh, how I wish I could remember who told me about the rabbits, go find him, and casually mention that!
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Old 12-01-2016, 09:53 AM
Isosleepy Isosleepy is online now
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Since Humans share DNA with Chimps and bananas, having two brothers sharing no DNA would mean one of them is some sort of alien experiment.

Having 2 brothers have no origin of genes in common is possible, but vanishingly unlikely.
Imagine you have a very long board with 20,000 numbered holes. You have 40,000 ping pong balls, 20,000 white, 20,000 black. All numbered. ( there's one black number 1, and one white -etc)
Now you start filling the holes, either a black #1 or a white, a black #2 or a white - till all holes are filled. Do this without looking! Now put all the balls back and do it again. What are the odds that the 2 arrays are completely different? My calculator rounds it to 0. If we were only doing this 2000 times, my calculator still rounds to zero, but i think the odds would be 1 to some number with about 91,000 zeroes in it, if it was 4000 times it'd have 8,281,000,000 zeroes in it, and after that my head starts hurting and we're not even to a fraction of what 20,000 times would be....

People with actual expertise in genetics and/or exponential math please correct the assuredly many errors above, but I hope I gave some idea as to how astronomically unlikely this is.
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Old 12-01-2016, 10:30 AM
Riemann Riemann is online now
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Since Humans share DNA with Chimps and bananas, having two brothers sharing no DNA would mean one of them is some sort of alien experiment.
Well, what you say is correct. But when we say "do two individuals share DNA" in the context above, it's implicit that we''re only considering the parts of the genome where any polymorphism* exists at all in the human population. Obviously at the bases where every human is identical, any pair of humans is identical regardless of ancestry.

*And, in fact, if we're considering inbreeding, we may only be considering the subset of human polymorphism that is not neutral, that has a phenotype, which will be far less than the total polymorphism.

Last edited by Riemann; 12-01-2016 at 10:33 AM.
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Old 12-01-2016, 10:32 AM
Leo Bloom Leo Bloom is offline
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I don't know if we actually share them in any transitive way, unless my Japanese botanical porn stream has been letting me down.
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Old 12-01-2016, 11:21 AM
septimus septimus is offline
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That's something like 1 chance in 64 trillion, as I make it.
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Originally Posted by Darren Garrison View Post
If no crossing over took place, the odds would be one chance in 8,388,608 (if I'm doing my math right.) With crossing over, you can add an undetermined number of zeros to that number.
If this is up for vote, I'll vote for chrisk's answer (substituting 70 for 64 1024≠1000), though adding in Darren's "undetermined number of zeros."
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Old 12-01-2016, 11:43 AM
md2000 md2000 is offline
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Mathematically, the odds are that any siblings share about 50% same DNA. The risk is that if one parent has a defective or weak gene, normally the OK version from the other parent will compensate. The danger in close relatives is that they may carry the same bad gene as one of their genes, and the child will get two copies of the bad gene and no good gene.

I guess the point is that unless there is a demonstrated problem, a family history of genetic problems -odds are there are very few genetic problems anyway. Many states and countries allow first cousins to marry, they have about 1/8 similar genes, so their child should only have about 1/16 "matched up" genes. Some studies show only about a 2% to 3% higher risk of genetic diseases for paired first cousins' offspring.
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First-degree relatives—parents, children and full siblings—will share about 50 percent of your autosomal DNA (atDNA). With each relationship removal, the expected amount of shared atDNA is cut in half. Second-degree relatives (grandparents, grandchildren, uncles, aunts, nephews, nieces, half-siblings) are expected to share about 25 percent of their atDNA. Third-degree relatives (first cousins, great-grandparents, great-grandchildren) will share about 12.5 percent.

As for more distant relationships: Second cousins share about 3.125 percent of your atDNA, and third cousins are expected to share about 0.781 percent. A cousin relationship that is once removed (separated by one generation) or shares only one common ancestor instead of a couple further reduces the expected amount of DNA matching by half.
we have 46 chromosomes, so 23 from each parent. Ignoring the shuffling that happens, simple math, the odds are 1 in 2^46. Consider 2^10 is approx. 10^3 (it's actually 1024, so close enough...) that's odds of almost 10^13, which is a big number, considering earth's total population is 7x10^9. Toss in crossovers and that adds an even bigger pile of zeroes at the end of those odds.
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Old 12-01-2016, 11:50 AM
Riemann Riemann is online now
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If this is up for vote, I'll vote for chrisk's answer (substituting 70 for 64 1024≠1000), though adding in Darren's "undetermined number of zeros."
Even discounting crossing over, the probability of "normal" chromosome assortment being identical twice is so small, that the real answer to this question is a much higher probability. The question is: what is the most plausible way that normal inheritance might mutate in a fashion that produced two siblings (which we can reasonably define as two people whose DNA comes entirely from the eggs & sperm of the same 2 parents) whose DNA is exactly complementary, and what is the probability of that. We certainly can't answer the question definitively, but mutations to meiosis that might have this effect seem plausible at a level many orders of magnitude below (1/2)^23.

Last edited by Riemann; 12-01-2016 at 11:54 AM.
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Old 12-01-2016, 11:51 AM
Darren Garrison Darren Garrison is offline
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we have 46 chromosomes, so 23 from each parent. Ignoring the shuffling that happens, simple math, the odds are 1 in 2^46.
One of us is misunderstanding the math, and I realize that it might be me.

My reasoning:ignoring crossing over, the chromosome chosen from each pair can be considered the same as a flip of a coin, or 23 choices between a "heads" chromosome and a "tails" chromosome. This would give 2^23 possible combinations of "flips", only one of which is "no duplicates", so 1/8,388,608.
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Old 12-01-2016, 11:52 AM
chrisk chrisk is offline
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If this is up for vote, I'll vote for chrisk's answer (substituting 70 for 64 1024≠1000), though adding in Darren's "undetermined number of zeros."
I actually got as far as the 8.3 million, realized I needed to square that, and rounded it down to 8 million first.
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Old 12-01-2016, 11:57 AM
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Even discounting crossing over, the probability of "normal" chromosome assortment being identical twice is so small, that the real answer to this question is a much higher probability. The question is: what is the most plausible way that normal inheritance might mutate in a fashion that produced two siblings (which we can reasonably define as two people whose DNA comes entirely from the eggs & sperm of the same 2 parents) whose DNA is exactly complementary, and what is the probability of that. We certainly can't answer the question definitively, but mutations to meiosis that might have this effect seem plausible at a level many orders of magnitude below (1/2)^23.
Actually, forget that. (1/2)^23 is only in the tens of millions, isn't it. Somehow I had it in my head that it was a much larger number.

Last edited by Riemann; 12-01-2016 at 11:57 AM.
  #29  
Old 12-01-2016, 12:12 PM
md2000 md2000 is offline
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One of us is misunderstanding the math, and I realize that it might be me.

My reasoning:ignoring crossing over, the chromosome chosen from each pair can be considered the same as a flip of a coin, or 23 choices between a "heads" chromosome and a "tails" chromosome. This would give 2^23 possible combinations of "flips", only one of which is "no duplicates", so 1/8,388,608.
So, the first child can be any combination at all - odds are 1. The second child must pick a specific combination to complement whatever the first child "picked". Each child has 92 chromosomes to "choose" from, 46 from each parent, 23 pairs each, and must pick 46 - i.e. from the first parent, pick A1 not A2 of the pair- and so on until they have 46. So like flipping a coin 46 times. For the "A" set (or whatever they're called) Whether you pick A1 or A2 from dear old dad does not affect whether you pick A3 or A4 of the matching pair from mommie dearest. One pair, two coin tosses. The first child, any pair will do. The second child has no choices, they must pick the opposite, and that's where the odds come in.
  #30  
Old 12-01-2016, 12:13 PM
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There is a real biological way that we can remove the additional requirement for unlikely probability of exactly the same crossing over happening twice on every chromosome.

In meiosis, the DNA a of a haploid (2N) cell is first duplicated to make a 4N cell where all 4 chromosomes align for crossing over. After crossing over, there is a first nuclear division 4N>2N. One of those cells gets one product of crossing over, the other gets the exact complement. The cells are still 2N, so there are two different chromosomes in each cell, each of which is complementary (with respect to crossing over) with one of the two different chromosomes in the other 2N cell. There is then a second division 2N>1N, in which the chromosomes assort randomly. So we still need the random assortment to occur identically in each 2N cell when it goes 2N>1N, but that is "only" a probability (1/2)^23.

The scenario above would only apply to fraternal twins, but we are now asking: what is the probability that, in fraternal twins, both sperm are products of the same meiosis, and both eggs are from the same meiosis? The sperm number would usually be very large of course (many sperm in one ejaculate), but we can envision spermatogenesis completing with two sperm somehow stuck together. I'd hazard a guess that maybe this is less unlikely than the probability that crossing-over happens twice identically in two different meioses.

So we're left with the (1/2)^23 factor for chromosome assortment, the probability of fraternal twins, and the probability that the two eggs and the two sperm that create the fraternal twins are the products of the same meioses.

Last edited by Riemann; 12-01-2016 at 12:17 PM.
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Old 12-01-2016, 12:17 PM
Darren Garrison Darren Garrison is offline
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Each child has 92 chromosomes to "choose" from, 46 from each parent, 23 pairs each, and must pick 46 - i.e. from the first parent, pick A1 not A2 of the pair- and so on until they have 46.
Yeah, that' s my problem--I was counting only one of the two parents.
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Old 12-01-2016, 12:20 PM
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Yeah, that' s my problem--I was counting only one of the two parents.
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Actually, forget that. (1/2)^23 is only in the tens of millions, isn't it. Somehow I had it in my head that it was a much larger number.
Yes, of course it is a much larger number isn't it, because it has to happen twice. So it's around 10^-14 rather than 10^-7.

So maybe I can reinstate my mutation hypothesis as more likely.

Last edited by Riemann; 12-01-2016 at 12:21 PM.
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Old 12-01-2016, 12:23 PM
Darren Garrison Darren Garrison is offline
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The scenario above would only apply to fraternal twins, but we are now asking: what is the probability that, in fraternal twins, both sperm are products of the same meiosis, and both eggs are from the same meiosis?
Well, that leads to the question if a secondary oocyte in humans can split into two viable eggs instead of an egg and a polar body...
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Old 12-01-2016, 12:41 PM
Folacin Folacin is offline
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Heinlein had this in one of his novels (Time Enough For Love?) - slave twins created in a lab, done as a stunt so they could be safely bred together.
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Old 12-01-2016, 12:46 PM
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For a description of how it could be done (probably horribly inaccurate!) see Heinlein's Tale of the Twins the Weren't in Time Enough for Love

Last edited by MarcusF; 12-01-2016 at 12:47 PM. Reason: Ninja'd!
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Old 12-01-2016, 02:16 PM
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Well, that leads to the question if a secondary oocyte in humans can split into two viable eggs instead of an egg and a polar body...
Yes, you are right. I misremembered oogenesis, the 4N cell yields only 1 egg and 3 polar bodies, right?
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Old 12-01-2016, 02:31 PM
Darren Garrison Darren Garrison is offline
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Yes, you are right. I misremembered oogenesis, the 4N cell yields only 1 egg and 3 polar bodies, right?
Yep.
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Old 12-01-2016, 02:43 PM
Chronos Chronos is offline
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I'm disappointed that it took 34 posts to get the Heinlein reference.
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Old 12-01-2016, 03:01 PM
Darren Garrison Darren Garrison is offline
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Yes, you are right. I misremembered oogenesis, the 4N cell yields only 1 egg and 3 polar bodies, right?
Some googling shows that some scientists/researchers/whoever believe that human polar bodies could in theory be rarely fertilized, but that there is no known example (or even a way of testing for it.) It is called "polar body twinning" or "half-identical twins."

(Bonus interesting article on polar bodies.)

Last edited by Darren Garrison; 12-01-2016 at 03:03 PM.
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Old 12-01-2016, 03:26 PM
md2000 md2000 is offline
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Yes, of course it is a much larger number isn't it, because it has to happen twice. So it's around 10^-14 rather than 10^-7.

So maybe I can reinstate my mutation hypothesis as more likely.
No, it only has to happen once. It does not matter what the first child is, just that the second one has to specifically pick 46 chromosomes that complement.

I.e. I flip a coin 46 times and record the sequence, whatever it is - HHTHTHHTT.... Now I have to flip another 46 times, and get exactly the opposite... TTHTHTTHH... 2^46

(again, ignoring crossovers).

I vaguely remember the twins in Heinlein, I just didn't see any great significance to it. I remember when I read it, may moons ago, my reaction was "who the heck would possibly care?" Especially when you need a DNA analysis lab (tech did not exist at the time of writing?) to verify it.

Last edited by md2000; 12-01-2016 at 03:28 PM.
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Old 12-01-2016, 03:33 PM
md2000 md2000 is offline
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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.
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Old 12-01-2016, 04:01 PM
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No, it only has to happen once. It does not matter what the first child is, just that the second one has to specifically pick 46 chromosomes that complement.
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.

Last edited by Riemann; 12-01-2016 at 04:03 PM.
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Old 12-01-2016, 06:37 PM
Peter Morris Peter Morris is offline
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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.
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Old 12-01-2016, 07:01 PM
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Something just occurred to me.
...without reading the thread!

See my post #30 and several subsequent comments, incl Darren re polar bodies.

Where are you getting "hundred million" from?

Last edited by Riemann; 12-01-2016 at 07:06 PM.
  #45  
Old 12-01-2016, 07:03 PM
watchwolf49 watchwolf49 is online now
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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 ...

Last edited by watchwolf49; 12-01-2016 at 07:03 PM.
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Old 12-01-2016, 07:09 PM
Riemann Riemann is online now
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Originally Posted by watchwolf49 View Post
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...
Not so. Siblings always get half each from mother and father.
The requirement is that two siblings get precisely complementary halves.
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Old 12-01-2016, 07:23 PM
Peter Morris Peter Morris is offline
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Originally Posted by Riemann View Post
...without reading the thread!
I skimmed it. I somehow missedc #30. My bad.


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Where are you getting "hundred million" from?
I think I heard somewhere that there are a hundred million sperm in an ejaculation.
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Old 12-01-2016, 07:30 PM
CookingWithGas CookingWithGas is offline
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Two siblings could be fraternal twins and safely have children if they didn't have any dangerous recessive traits. In-breeding doesn't create genes, it just shuffles them around, just like sex between two unrelated people; the difference is that in-breeding has a higher chance of making otherwise-unusual combinations.
What is special about fraternal twins? How is that different genetically than any other pair of siblings?
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Old 12-01-2016, 07:30 PM
Riemann Riemann is online now
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I think I heard somewhere that there are a hundred million sperm in an ejaculation.
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.
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Old 12-01-2016, 07:33 PM
Riemann Riemann is online now
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Originally Posted by CookingWithGas View Post
What is special about fraternal twins? How is that different genetically than any other pair of siblings?
There is no difference, I think Derleth just mis-phrased what he was saying.
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