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.

(note: comments do not require an email.)

12 Comments

  1. Lingual trivia on this topic:
    In Arabic all these cousins are differentiated, using what’s a pretty easy to remember system (unlike Chinese, which is a nightmare).
    The building block words:
    Son:
    ibn = ابن
    Daughter:
    bint = بنت
    Paternal sibling:
    ʿamm/a = عم/ة
    Maternal sibling:
    khal/a = خال/ة

    So, a cousin is simply a son or a daughter of a maternal or paternal uncle or aunt (the female parental siblings are the ones with the ة [a] tacked on). So, from your diagrams, I’ll list the cousin names, first in Arabic script running from the right to the left and then the transliteration from the left to the right (the order of the cousins follows the direction of the script…sorry if that’s confusing, there’s really no non-confusing way of running opposing directional scripts in lists)

    ابن عمة, بنت عمة, ابن عم, بنت عم, ابن خالة, بنت خالة, ابن خال, بنت خال
    bint khal, ibn khal, bint khala, ibn khala, bint ʿamm, ibn ʿamm, bint ʿamma, ibn ʿamma

    I realize that if you haven’t learned the Arabic alphabet, the script looks like nonsense (and that if you have learned Arabic, this isn’t new information), but I thought that it might be interesting (and pertinent) trivia.

    Reply

  2. thnx, meng!

    i would’ve guessed that the cousins were differentiated in arabic — they are, as well, in other languages in societies where the regular marriage of cousins takes place — but i didn’t know for sure.

    i self-taught myself the arabic alphabet several years ago (don’t recall much of it now!) in order to be able to identify words on street signs and the like. i didn’t actually learn any arabic, though! (~_^)

    Reply

  3. @luke – “I’ve not seen that analysis before. Is it original with you?”

    well, the analysis above is something i have done. if somebody else has done something similar with regard to cousins, i haven’t seen it.

    the only piece of research i’ve seen with a similar “train of thought” is one that i linked to above: Grandma plays favourites: X-chromosome relatedness and sex-specific childhood mortality.

    in that study, the researchers took a look at the grandmother effect, taking into consideration how related maternal and paternal grandmothers are to their various male and female grandchildren (with regard to the x-chromosome). the relatedness differs in an, apparently, not insignificant way.

    cool stuff! (^_^)

    Reply

  4. Hi HBD Chick,

    I stumbled across your post on the net. I do have a question… Does this mean that patrilateral parallel cousins (each of the opposite sex) are at more risk of inbreeding? Or is their pairing the least likely to have the same genes?

    Reply

  5. @sweetdelight – hi there! well, since i don’t know in what context you’re asking this question (like if you’re thinking of marrying a cousin), before i answer i want to say that i am NOT an expert on anything genetic. i’m not a doctor — i’m not a geneticist. i just mess around on the internet. (~_^) so, if you’re looking for an answer to this question on medical grounds, please ask a professional.

    having said that, by my calculations (which may or may not be right), probability says that a man should be equally related to his patrilateral parallel cousin (his father’s brother’s daughter or fbd) and and his patrilateral cross cousin (his father’s sister’s daughter). (also two of his male cousins, but he can’t reproduce with them — not naturally anyway).

    these are actually his two females cousins with whom he probably shares the least genes. he shares the most with his mother’s sister’s daughter.

    remember, these figures are averages or probabilities, not definite amounts. the only definite amounts of shared genes are between parents and children — we all definitely get half of our chromosomes from our mother and the other half from our father (most of the time).

    Reply

  6. Can you give an explanation of how these relatedness issues might play out in matrilineal/matrilocal systems?

    Reply

  7. @msharmila – “Can you give an explanation of how these relatedness issues might play out in matrilineal/matrilocal systems?”

    ooo. you know — i don’t know! i’ve never thought it through.

    let me think about it for a few days (a week?), and i’ll get back to you on this. (might even write a post about it.) i’ll let you know when i’ve got something for you (or if i don’t come up with anything at all!) — i know where you live your blog is. (~_^)

    in the meantime, you might find these posts interesting: tribes and types of cousin marriage and tribes and types of cousin marriage ii.

    good question! thanks! (^_^)

    Reply

  8. Help with this puzzle would be so appreciated. I was recently trying to determine my father through autosomal dna testing. I am female. There are two possibilities and they are brothers. I tested their sister’s child and she came back a first cousin 12% match in dna. I tested one of the brother’s daughters and she came back a half sibling 26% match. Given what you say here about the X Chromosome,

    My question is could female patrilateral parallel cousins look like half-siblings genetically?
    And my second question is what is the likelihood statistically?

    Reply

  9. So I’m trying to figure out which if any DNA tests will confirm that my 2nd cousin and I are indeed 2nd cousins without involving my biological father, her father’s cousin. According to your explanation I would basically have the same Y Chromosome as her father, but our X Chromosomes would only have a 12.5% match? Or would that be a 25% match?

    Reply

  10. All this analysis is only for the sex chromosome. That is just one of 23 pairs. For the rest 22 the child gets a combination of both parents. So the consanguinity is same for cross or parallel cousins. In some cultures cross cousin marriage is encouraged but parallel cousin marriage is discouraged (e.g. South India)
    That is because the chances of a parallel cousin being a sibling is higher (Father could have mated with brother’s wife).
    Both cross or parallel cousin marriage is deemed incest in most of civilized world these days. It is even a crime in parts of US

    Reply

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