Me & my 0.55 brother against my 0.45 brother

By Razib Khan | September 4, 2012 11:41 pm

One of the more fascinating things about getting much of your child’s pedigree genotyped is that one can ascertain true relatedness to various relatives, rather than just expected relatedness. For example, 28% of her genome is identical by descent from my father, while 22% is from my mother. She is 26% identical by descent with one uncle, and 24% with another. More practically, the understanding of patterns realized and concrete genetic relatedness within families allows us another avenue into teasing apart heritability. Though this method has been around for more than half-a-decade, I find it curious that when I post on it some commenters immediately make objections to twin studies. Why? Because they assume that the analysis had to be a twin study because they don’t know of the genomic methodology!

But on a broader evolutionary scale, does this matter? Two of my siblings have a relatedness of 41%. In other words, as you can see in the histogram there is a wide variation in relatedness. Might this perhaps impact social relations? One can imagine genetically more similar siblings aligning against those who are dissimilar. Or not. I am skeptical that this would apply to humans, but I do wonder about organisms with larger broods. If we don’t find much variation on the scale of siblings, despite genetic variation (and therefore, likely phenotypic tells of similarity), then I would hazard to suggest that inclusive fitness is not quite the razor sharp discerning tool that some posit it is. Rather, it is part of the broader swiss army knife of behavioral ecology.

MORE ABOUT: Genetics, Heritability
  • April Brown

    This really makes me wish I could convince all the relatives my husband and I have in our parents’ generation to do DNA typing – I think this would be really neat way for our kids to think about where they came from (especially because we move every two years to another part of the world, which is rough on a kid’s sense of place and belonging). Alas, most of them find this sciency stuff to be irrelevant or sinister, depending on how republican they are.

  • Karl Zimmerman

    Out of curiosity Razib, is the brother you share more DNA with the brother you have similar politics to?

    Assuming that genes which affect personality traits are fairly evenly distributed around the genome, overall genetic similarity should track closely with personality similarity between siblings. And studies have suggested that friendships tend to form between those who have similar levels of Agreeableness, Extraversion, and Openness. Hence, it could be a valid hypothesis that sibling relationships would also be stronger among those sharing a greater than average amount of DNA.

  • M. Möhling

    OT, @April:
    > we move every two years to another part of the world,
    > which is rough on a kid’s sense of place and belonging
    From personal experience I strongly advise against it, so does a Canadian study. I enjoyed moving and traveling as a child, but had to find out later that it takes a toll.

  • Jeremy Leipzig

    It is considered good form to cite your figures,

    Visscher PM, Medland SE, Ferreira MAR, Morley KI, Zhu G, et al. (2006) Assumption-Free Estimation of Heritability from Genome-Wide Identity-by-Descent Sharing between Full Siblings. PLoS Genet 2(3): e41. doi:10.1371/journal.pgen.0020041

  • Julia

    Intersting, but of course, the 0.55 only describe the probability of having sequences in common, but wether a certain gene or not is present would be what counts insuch a sibling rivalry situation. Or, posed differently, the approximation of relatedness derived from a pedigree is an average for all genes or QTLs, while in this context, one has to look at single QTLs or genes and their “relatedness” is always either 1 or 0. I hope this makes some sense.

  • Razib Khan

    #4, thanks. forgot to put the links up. that’s what i usually do.

    is the brother you share more DNA with the brother you have similar politics to?

    no. in fact, the one i physically resemble the most (and personality too) has slightly less relatedness. though the difference is not much.

    one has to look at single QTLs or genes and their “relatedness” is always either 1 or 0. I hope this makes some sense.

    i think sibling rivalry would be due to polygenic effects. so i think overall genomic similarity is probably more important.

  • pconroy

    Of my 3 kids, they are like their grandparents in the following proportions – overall:

    1. PGF 62%, PGM 38%

    2. PGF 47%, PGM 53%, MGF 55%, MGM 45%

    3. PGF 46%, PGM 54%, MGF 42%, MGM 58%

    Where P=Paternal, M=Maternal, GF=Grandfater, GM=Grandmother

    For specific traits like Immune System genes, they are:

    1. PGF 100%, PGM 0%

    2. PGF 0%, PGM 100%, MGF 0%, MGM 100%

    3. PGF 0%, PGM 100%, MGF 0%, MGM 100%

    So somehow my 2 youngest have an entirely different immune system that my eldest??

  • BDoyle

    I might be tempted to call inclusive fitness more of a blunt instrument rather than a Swiss Army knife. How well could someone actually estimate from phenotype how genetically similar he is to a sibling? Might be interesting to do a test and find out. In practice, I think that kin preference would have to rely on some easy-to-apply proxy for relatedness (like being raised in the same litter), that *usually* works, rather than some detailed process to determine precisely how close you are genetically.


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About Razib Khan

I have degrees in biology and biochemistry, a passion for genetics, history, and philosophy, and shrimp is my favorite food. In relation to nationality I'm a American Northwesterner, in politics I'm a reactionary, and as for religion I have none (I'm an atheist). If you want to know more, see the links at


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