all cousins are not created equal

here’s a chart of standard cousin relationships (charts adapted from here). as you can see, there are four different types of cousins – cross and parallel cousins, both patrilateral and matrilateral:

now it may seem that, apart from being different sexes (triangle=male, circle=female), there’s not much difference between all these cousins, but there is. there are differences in how both the x- and y-chromosomes get passed down in families, and these differences show up in one’s cousins.

first, the y-chromsome — the chromosome that makes men men — passed down virtually unchanged from fathers to sons:

notice that ego shares his y-chromosome with his father (of course) and his brother — AND his father’s brother (i.e. his paternal uncle) AND his father’s brother’s son (i.e. his male paternal cousin). as a group, they are really an inter-generational “band of brothers”, so to speak.

now the x-chromsome.

everybody gets one x-chromsome from their mother, and women get a second x-chromsome from their fathers. the x-chromsome that you get from your mother is a recombined hodge-podge of her two x-chromosomes. the x-chromsome that a woman gets from her father is, like the y-chromosome he passes on to his sons, virtually unchanged.

so, here’s the x-chromosome inherited in the father’s line:

notice that all of a man’s daughters inherit his x-chromosome virutally unchanged.

also, the father’s brother and sister (ego’s paternal uncle and aunt) have inherited an x-chromosome similar to the one that ego’s father has. they are not, however, exactly the same. the three of them have inherited one recombined x-chromosome each from their mother (ego’s paternal grandmother — not indicated on chart). so, while they share some genetic material with each other on their x-chromosomes, the chromosomes are not identical. (ego’s aunt has also inherited one virtually unchanged x-chromosome from her father, ego’s paternal grandfather, not indicated on chart.)

any daughters of the father’s brother will inherit a virtually exact copy of his x-chromosome. so, those cousins’ x-chromosomes (the one they inherit from their father) will be just as similar to their uncle’s (ego’s father) as his brother’s is. (confused yet?!) for instance, if the father and his brother share 50% of the genes on their x-chromosomes, then the father’s brother’s daughters will also share (nearly) 50% of their genes on their x-chromosomes with their uncle (ego’s father). (got it?)

following from that point, ego’s female patrilateral parallel cousins and his sisters will share nearly the same amount of genetic material on their x-chromosomes as the cousins do with their uncle, since ego’s sisters inherit a virtually exact copy of ego’s father’s x-chromosome. (say THAT five times fast!)

ego’s patrilateral cross cousins — the children of ego’s father’s sister — inherit one recombined hodge-podge of their mother’s x-chromosomes each. so, while they do share some genetic material on those x-chromosomes with ego’s father and ego’s sister, it is not such a close relationship as any patrilateral female parallel cousins. it is closer, though, than with any patrilateral male parallel cousins. neither of ego’s patrilateral cross cousins share any genetic material with ego’s x-chromosome, which he inherited from his mother (cue next chart).

so, ego’s mother and her brother and sister (like ego’s father and his brother and sister) share similar genetic material on their x-chromosomes; but, since their x-chromosomes are recombined versions of their mother’s (ego’s maternal grandmother – not indicated on chart) x-chromosomes, their x-chromosomes are not identical to one another.

ego and his brother and sister have each inherited one recombined x-chromosome from their mother. those x-chromosomes are similar, but not identical.

ego’s mother’s sister’s children have each inherited one recombined x-chromosome from their mother. they share some genetic material on those x-chromosomes with ego, but not as much as any of his female matrilateral cross-cousins do. those female cousins inherited an almost exact copy of their father’s x-chromosome (ego’s maternal uncle). they, therefore, probably share a greater amount of genetic material on those x-chromosomes with ego. ego and his male matrilateral cross-cousins do not share any genetic material on their x-chromosomes since those cousins do not inherit an x-chromosome from their father.

whew! got all that?!

i only bring all this up because it has bearing on what i was babbling about in “cousin marriage conundrum addendum“, i.e. that the type of cousin marriage (inbreeding) also matters, not just the coefficient of inbreeding (or kinship or whatever), when we’re discussing the “innate social apptitudes of man.”

i also wanted to write all this out to help myself get a grasp of all these relationships. (it ain’t easy!) let me know if you see any errors.

one of these days i’ll terrorize ya’ll with another post(s) showing what happens when all these different sorts of cousins mate. (don’t say i didn’t warn ya!)

see also: Probing Question: Do sisters share a closer genetic proximity than other siblings? and Grandma plays favourites: X-chromosome relatedness and sex-specific childhood mortality

previously: cousin marriage conundrum addendum

update 03/10: i tweaked some of the wording in this post to try and make it a bit clearer. (don’t know if i succeeded or not!) none of the changes were substantial.

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