If identical twins were to marry identical twins, then what would their children be to each other?

Genetically, the children would be siblings if twins married twins.

@gailcalled: Oh… so there would probably be some genetic defects in there somewhere, right?

They would be cousins, and that is it. The DNA may be similar, but it isn’t exact. Even identical twins have different fingerprints.

I don’t think DNA would work like that being that eggs and sperm carry any number of genetic traits.
There will be no guarntee that they would have the same identical eggs and sperm. However they would probably be as close as siblings. But one can have traits from one family be dominate and the other have traits from the other family.
Brothers and sisters have the same genetics and still come out looking different.

so there would probably be some genetic defects in there somewhere, right?

No, they would simply be more similar to their cousins than most.

They would be genetic siblings, but cousins when describing the familial relationship. They are not more likely to have genetic problems, it is not two related people making a baby, the parents of each child are not siblings.

If you ran a DNA test on the children, you would not be able to tell which couple was the parents.

Cousins

What if both couples had twin girls. Would that make them Genetic Quadruplets?

@Ltryptophan Here you go.

@JLeslie I trust your opinion better than mine on this. Do you have any reference to support that the children would technically be siblings?

Identical twins usually have identical DNA (but there are some cases of the DNA mutating and being slightly different). Although identical twins have the same DNA, they have different phenotypes, meaning that the same DNA is expressed in different ways.

The easiest way I can explain the genetic part for cousins is this: cousins (regular cousins) would have about half or less of the same DNA as the other normally. In the case of twins marrying twins, the cousins would have much more than half the same DNA (closer to it being the exact same) making them siblings in a genetic sense.

For example:
Jim and John are identical twin brothers. They marry Sue and Nancy who aren’t related. Jim and Sue have a child named Billy and John and Nancy have a child named Jack. Billy and Jack would get the same DNA from their dad’s but different from their mothers, so their DNA would only match half way (less if the brothers weren’t identical twins).

Now Nathan and Shawn are identical twin brothers that marry identical twin sisters Sally and Grace. Each has a child, Allen and Eric respective. Since Allen and Eric would get the same DNA from their dad’s and then the same DNA from their mom’s, genetically their DNA would match (or almost match considering there could be slight variations) and they would be siblings genetically while legally being cousins.

The siblings mating leading to birth defects would not come into play because the child is getting half of their DNA from one family and the other half from another family. If the parents were brother and sister, that could lead to possible birth defects.

@Pied_Pfeffer not a reference just seems logical. Each set of parents are identical.

@Ltryptophan that would be impossible. It is the same as saying two siblings randomly being identical genetically from different eggs and sperms. I can’t imagine that could ever happen.

@lillycoyote that’s good.

@all I don’t understand why this question is confusing? Is it because people don’t have a basic understanding of DNA? I don’t mean that in a condescending way, plenty of things I don’t know, just curious why people are having a tough time with thinking it through.

@JLeslie Parents only pass on half of their genes, and it’s impossible to tell which half. So twin couple A might pass on one half of their genes, and twin couple B might pass on the other half, and then genetically they’d be cousins same as everyone else. And the reason it’s really hard is because pundit squares (is that what they’re called?) are insanely hard to do in your head.

Legally the offspring would be cousins. Genetically they’d be siblings. The risk of genetic defects would occur only if the offspring decided to get together and have a child.

Genetically, they would NOT be cousins. And there would be no genetic defects because of this.

Here is an article that supports those saying that the offspring would genetically be siblings:

In species that pass down their DNA sexually, the gametes of the father unite with the gametes of the mother. The kicker is, the offspring get exactly half of their DNA from each parent. Which half? Richard Dawkins, in _The Blind Watchmaker, says the process isn’t exactly a coin flip, but “can be treated as random…. Every sperm produced and every egg produced is unique in terms of the contents of their locations [i.e. the makeup of which nucleotides – or DNA particles – go into which chromosomes].” The upshot of this is that you are exactly 50% related, genetically speaking, to your father and 50% to your mother. Simple probability shows that you are also 50% related to a brother or sister—in other words, the two of you are likely to have 50% of your genetic material in common. Except for an identical twin, to whom you are 100% (except for some typical, meaningless mutations) related._

Now, look at the second generation. I am 50% related to my brother. Our wives are not related. Therefore, my kids get ‘diluted’ by a factor of ½, and are ¼ related to him. His kids are, of course, ¼ related to me, and therefore 1/8 related to their first cousins, my kids, all due to the aforementioned probability. Now, what if we were twins? My gametes, which contain exactly ½ of my genetic material, would also contain ½ the genetic material of someone who shared my DNA. That is, since we are identical in every way, one of us can be freely substituted for the other in this little tree. My kids are still ½ related to me, but now you can see that they are also ½ related to their uncle! Similarly, his kids are ¼, rather than 1/8, related to my kids; the inclusion of one set of twins in the family tree doubles the genetic relationship of the parties involved. If our wives are also twins, the ‘relatedness’ is doubled once again, so that my and my wife’s kids would be ½ related to my brother’s and her sister’s. Now, we saw in the last paragraph that brothers and sisters share 50% of their genetic material, and in this special case, our kids, first cousins, would be just as related to each other as if they were siblings.
Source

@papayalily the mother is identical to the other mother, basically interchangeable genetically, so is the father with the other father. The genetic sequence given to a child of one of the couples is half from each parent. When the very same couple has another child, there is a new random sequence. Every time there is a new baby created from the same couple there is a new random sequence, so when the identical couple creates a baby from their own DNA, which is exactly the same as the other couples DNA it is the same as the first couple producing another baby.

If a scientist tested the DNA of one of the children, and the four parents, the scientist could not tell you which twins were actually the real parents.

@papayalily just to go a little further, you say a parent passes on half, but each time the “half” is a different sequence. Let’s say your mom has genes for brown and red hair, and blue and brown eyes, and thick hair and thin. When her DNA splits to give her baby half, sometimes it will be red hair, blue eyes, thick hair. Another time brown hair, blue eyes, thick hair. Another time brown hair, brown eyes, thick hair. But, this is for hundreds of traits, the combinations are almost endless. (that example is very simplified) It is not like the mother gives out the same half of her DNA every time, otherwise we would have lots of twins walking around who are a few years apart within one family from the same parents.

It’s an interesting question. Genetically, there is a incredibly slight possibility that the two kids would have identical genotypes, (the same possibility that common siblings would have the same genotype) and would most likely resemble each other as much as siblings. They aren’t exactly siblings, but they are they both have DNA that was drawn, though in different measures, from identical sources.

I’m not sure what “siblings” means in a genetic sense, since the word and understanding predates understanding of DNA, but when it comes down to the potential for genetic defects, these two kids should certainly never attempt to procreate.

They would all be cousins but they would look like brothers and sisters.

I learned this tidbit, informally, from a set of 18-year-old twins I worked with. Their doctors told them that.

Just like true siblings, they might look astonishingly alike or very dissimilar.

Right, @gailcalled is correct. They are related genetically like any true siblings, and might look very alike or not, but are still genetically siblings.

@JLeslie : I hope they don’t have any baby mama drama, then!

@troubleinharlem I don’t understand what you mean.