How related will the children of two sets of identical twins be?

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Offline chris

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If a set of identical twins (call them AA) marries and has children with another set of identical twins (call them BB) - so we have A-B + A-B - how related to each other - genetically - will the children be?
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Offline CliffordK

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First generation cousins would be almost indistinguishable from being full siblings.

Each parent may carry a couple of novel mutations which could be passed to the children, but these may only be detectable with 100% genetic sequencing. 

Obviously there would only be any second generation identical twins from the same mother, so you could not have identical cousins, without having in vitro fertilization from a single split blastocyte.


Offline kingbred

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Indeed the 'cousins' will genetically resemble full-sibs, bar the few mutations that have taken place in the germline of the 4 parents.
Phenotypically they are likely to differ more than 'regular' full sibs because they will have a different maternal environment, both pre-natal and post-natal.

However, the actual full-sibs, i.e. kids born from the same couple will have the same genetic similarity as normal full-sibs: The fact that both their parents are half of a pair of monozygotic twins only affects their genetic similarity with cousins that are chicldren of the other half of the twins.

Of course any of the couples can have a pair of monozygotic ('identical') twins themselves and it is an interesting question whether there is a genetic dsiposition for having monozygotic twins. While both monozygotic and di-zygotic twins are used  extensively in human research I have no quick answer whether there is genetic disposition to monozygotic twinning. There is some evidence for a genetic disposition for dizygotic twinning. The latter is more plausible because it is related to ovulation rate while the former is more a process related to embryo development.