Topic: Switch case smaller than a second
July 17, 2019 / By Em Question:
If two twin sisters each married a brother from another set of twins,
would both of their offspring carry enough of the same DNA to be considered brothers/sisters instead of just cousins?
I thought about this in the car the other day and thought it was neat to think about..
Thanks you guys!
I do know that "brother/sister" and "cousin" is just a name for a certain place in the family tree,
but my question was just asking as too whether the D.N.A would be more alike in a brotherly/sisterly way than the way cousins share blood.
Christie | 7 days ago
A similar question to this was on yahoo answers 4 months ago and was also asked on http://www.thenakedscientists.com last December.
Here's the answers given there on yahoo and the naked scientist, all of which are good answers with great explanations.
when sex cells form, the chromosomes pair off (you have 23 pairs, meaning 23 chromosomes came from your mother, and then you have 23 chromosomes from your father), and then they break apart and the pieces recombine. So, say 'A' is one chromosome strand (the chromosome from the person's mother), and 'B' is another chromosome strand (from the person's father. They would break apart and switch with each other so it would look like this:
This happens over and over again, until the chromosomes eventually look like this:
See? Now the chromosomes are different. So, between two people, ALL the chromosomes would get mixed up over and over again, to the point where certain traits would be displayed that were hidden in the individual, and the children could look very different. Chances are, they would all look similar, but you can never really tell how recombination is going to work (as where the chromosomes break is completely random), especially among four different people.
One last thing to consider: there are millions upon millions of genes on each chromosome. So, they would be MUCH longer than the 'A' and 'B' chromosomes I just showed you, which means everything is opened up to even more chance happenings!
Answers from naked scientist.
1) With the exception of identical (monozygotic) twins, triplets, etc, each child receives a different mixture of genes from each parent,
(if this were not the case all siblings would be identical).
So the children of one couple would not be identical to the children of the other couple.
Genetically these cousins would be like siblings, so they would be unusually similar cousins, but not identical.
2) Interesting question! Intuitively, you can see the answer is likely to be "no". If the answer to your question were an unconditional "yes", wouldn't two brothers always be the same? We know this isn't true. You basically have two identical couples each producing offspring, in the situation you describe. From a genetic standpoint, this is absolutely no different than having one normal couple producing offspring. This is very important to realize.
More rigorously, you have 46 chromosomes, or 23 pairs. You get 23 chromosomes from dad, and 23 from mom. But each chromosome you receive from mom, is randomly chosen from 2 chromosomes (from each of her pairs). So the probability of two siblings receiving exactly the same chromosomes from mom would be (0.5)^(23) = an extremely small number. This is the probability that two kids in your situation would be identical. Or 0.5^22 if we assume they are of the same gender. With this assumption, the probability is about 1 in five million.
This answer states that the probabilty would be about 1 in 5 million so that means that no matter how remote, in theory, it is therefore possible for the offspring of both couples to be genetically as close as siblings!
No, the offspring would still just be cousins. Because the parents are different DNA strains. While their odds are better then average that they would have twins or more. Since the twin gene is no longer recessive but it is now dominate or fairly dominate. It is a good question, though never thought about it on a genetic level.
If identical twin sisters married identical twin brothers then their children, while being cousins, would genetically be the same as siblings. If the sisters/brothers were not identical then I think the genetic similarity would be somewhere between cousins and siblings.
The correct term is actually 'Double first cousin'.
And to answer your question, the DNA is closer than normal cousins, but not quite as close as actual siblings.