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Question: If you had identical twin guys and identical twin girls and they had kids, would their kids look the same?
Answer: The short answer is yes; these cousins would look just as much alike as two naturally related brothers or sisters. The reason they look similar but not identical is a little more complicated.
Each person starts with 23 pairs of chromosomes that contain DNA, one member of each pair from their mother and one from their father. These are provided at conception, the DNA from the mother in the egg and the DNA from the father in the sperm. Identical twins start out from a single fertilized egg that begins dividing and early on divides so that it splits into two separate embryos, both with the exact same DNA or chromosomes. The DNA information is what determines your eye color, the color of your skin and in part how tall you will be. Some of that DNA information comes from your father, so part of what you look like comes from him. But another part comes from your mother, so you look like her a bit as well. Brothers and sisters look alike because they get similar DNA information from the same parents. But they are very rarely identical because the DNA information can come from one of the grandparents and not the other.
Let me explain another way. Say the two identical boys above are named John and Jacob and the two identical girls are named Kate and Karen. Let's take the first chromosome pair from John. He got one of the pair of this chromosome from his mother and one from his father (let's call the chromosomes 1M and 1F). Because he's identical, Jacob has the same pair of chromosomes. Kate also got a pair of the first chromosome, again one from her Mom and one from her Dad (let's call these 1m and 1f; lower case). Karen has the same. Most of these 4 different first chromosomes are identical but there are some differences between them that explain why each person on the planet doesn't look identical.
Anyway, when Kate makes eggs, some of them will have the 1m and some will have the 1f (none will have both). John makes lots of sperm that either have 1M or 1F, and it's a random event as to which type of sperm actually fertilizes the egg.
So there are 4 different possible combinations of this first chromosome in fertilized eggs from John and Kate, 1M1m, 1M1f, 1F1f and 1F1m. Depending on the DNA in each of these chromosomes these actually might look like different people, but mostly they have a good chance of looking at least similar, or related.
That's only 1 of the 23 pairs of chromosomes in people, so there are lots of different combinations. You probably know that even sisters and brothers can look very different although most of the time they look like each other. So in the question above, the two boys (John and Jacob) have the same chromosomes or DNA, and the two girls (Kate and Karen) have the same chromosomes or DNA. Thus, each couple (John with Kate and Jacob with Karen) is basically identical and therefore these couples are like one couple who have multiple kids. So, the children from each couple should look just as similar as two children from the same parents.
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