>Brainlets in this fucking thread
Did you even study biology at school?
During meiosis, chromosomes are randomly and independently assorted at the equator of the cell before the cell is split in half twice, forming 4 genetically distinct gametes. Due to there being 23 pairs of gametes, a person can create gametes with 2^23 (8,388,608) combinations of chromosomes. This would be squared when you take into account a zygote is made of a gamete from each parent. So there are 70,368,744,000,000 combinations of the two gametes.
Additionally, before splitting, each homologous pair of chromosomes will touch and swap genetic material between non-sister chromatids in a process called 'crossing over'. This always happens, and swaps genetic information from paternal chromosome with maternal. This means even if a gamete happened to have entirely paternal chromosomes (1/8,388,608 chance) they would all have some maternal DNA, due to crossing over. So there is an unfathomable number of ways the DNA can be swapped between non-sister chromatids, between 23 homologous pairs in 2 different people.
If we are to simplify genetics and treat these two pairs of married twins as the same couple, as well as removing any chance of aneuploidy causing issues (e.g. Down's syndrome), anticipation of trinucleotide repeats causing issues (e.g. Huntington's and Fragile-X), genetic imprinting causing issues (e.g. Angelman's & Prader-Wili) and also ignoring the fact that crossing-over happens:
There's a 1 / 70,368,744,000,000 chance that the same combination of gametes could happen in the two couples.
But, if you don't ignore all that stuff above, then the answer is unfathomably small.
