so, i found out that mother’s brother’s daughter (matrilateral cross cousin) marriage is, apparently, the most common form of cousin marriage globally.

i looked at father’s brother’s daughter marriage before, and noted one interesting feature in which ego’s paternal uncle (his father’s brother) got to pass his y-chromosome on to his daughter’s male children when she married her paternal cousin (because ego shares his paternal uncle’s y-chromosome). this is something a man wouldn’t normally be able to do (since his daughters don’t inherit his y-chromosome). so, fbd marriage is a really good deal for the paternal uncle.

what happens in mother’s brother’s daughter (mbd) marriage?

well, first of all, the y-chromosome doesn’t really matter here because ego doesn’t share a y-chromosome with any of his maternal relatives (unless everyone’s really inbred!).

in the case of x-chromosomes, ego inherits just one x-chromosome from his mother — an x that is a recombination of her two x-chromosomes. so, let’s track her two x-chromosomes.

in the case of ego’s mom’s maternal x-chromosome (i.e. the one she inherited from her mom), ego’s mother’s brother shares an x-chromosome similar, but not identical, to the one that ego’s mom has. (each of them inherited a recombined x-chromosome from their mother.) he passes an (almost) intact copy on to his daughter (the mbd), while ego gets part of his mom’s maternal x-chromosome recombined. the result looks like this (click on images for LARGER versions):

so there’s a few bonuses for several individuals here:

– some of ego’s x-chromosomal dna passes on to ego’s son via ego’s wife (the mbd) — normally a male doesn’t pass any x-chromosomal dna onto his sons;
– ego’s daughters get an extra amount of his x-chromosomal dna via their mother — another way of looking at it is that the mbd gets to pass on an extra amount of her x-chromosomal dna via ego;
– both ego’s mom and his maternal uncle get extra amounts of their x-chromosomal dna in their female grandchildren, and ego’s mom gets to pass some of her x-chromsomal dna on to her son’s male children, something she normally wouldn’t have been able to do.

ok. so that was one of ego’s mom’s x-chromosomes. what about the other one? the other one she inherited from her father, so she doesn’t share it with her brother. so it looks like this:

not very exiciting.

buuuuuut, what if ego’s dad had also married his mbd? that’s how they do it, ya know.

then it would look like this:

as you can see, ego’s father would also get to contribute some of his x-chromosomal dna to his female grandchildren’s genomes (via ego’s mother), something he wouldn’t have done otherwise.

so, that’s a short summary of what happens with the x- and y-chromosomal dna in mother’s brother’s daughter marriage. let me know if you see anything else of interest — or if you see an error in something i wrote here (i am experiencing a bit of a cupcake-induced brain fog after today’s festivities (~_^) ).

previously: father’s brother’s daughter marriage and coefficients of relationship – cousin marriage – grandparents

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