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Genetically more related to first cousin than sibling?

  1. Sep 22, 2013 #1
    This question started with the asian culture where cousins on the father's side is "closer" than cousins on the mother's side.

    If we have three siblings, brother 1, brother 2 and sister. let's say, b1, b2 and s3. They each have one child, c1, c2 and c3. The asian culture would be that c1 is more closely related to c2 than c3.

    How i understood it was, b1, b2 and s3 each has inherited half from their mother and half from their father, let's say m and f; c1, c2 and c3 has inherited half from b1, b2, and s3 respectively, then each of c1, c2 and c3 (the grandchildren) has 1/4 of the DNA from each of m and f (the grandparents). Therefore, c1, c2, and c3 should be genetically related to each other at the same degree.

    However, after i reviewed meiosis i remembered that chromosomes come in pairs and they undergo recombination before the cells split. Let's say s3 actually has two children, c3 and c4. c3 has inherited all the 1 genes from mom, and all the 1 genes from dad, but c4 inherited all the 2 genes from each parent, all of a sudden we have siblings that are opposite to identical twins. While that could happen, the cousins only need to have inherited one single common gene from the grandparents to be more genetically related to the siblings. Likewise, the cousins c1 and c3 can be more genetically related than c1 and c2.

    Am i right about the siblings and cousins? Please comment and educate.
  2. jcsd
  3. Sep 22, 2013 #2


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    Apart from X/Y chromosomes, the sex does not matter. For each chromosome pair, it is random which chromosome the child gets.

    b1 and b2 will have the same Y chromosome (from their father) and a random X chromosome from the mother, if their children are both sons, those sons will have the same Y chromosome as well, giving more similarity. But this is negligible compared to the randomness of the other chromosomes, and it applies only to the case where both brothers have sons.

    Cultural traditions are rarely coming from genetics.
  4. Sep 23, 2013 #3

    Genetic relatedness is well defined whereas we don't even know what those 'Asian' people whom you refer to, mean they say one cousin is more related than the other. I'd reccomend reading this page before you consider this.
  5. Sep 23, 2013 #4
    I am not seeing how you can make such a conclusion as one is more to related to cousins than their sibling. All of us receive a set of chromosomes from each parent. Genetic related ness is a vast topic and very complex one. Only in cases of gene disorders (single gene)(eg. hemophilia) will inheritance of gene can be deduced easily. As far as relatedness there are many protein(via RNA) and non protein coding genes in our chromosomes. All these put together with the effects of crossover during meiosis makes it difficult .

    However in population studies scientists often use mitochondrial DNA or y chromosome since it has lesser base pairs/ genes to study.
  6. Sep 23, 2013 #5

    So I read this "relatedness tutorial" online: http://ess.nbb.cornell.edu/relatedness.html

    For every gene of a human, there are a set of two alleles and each combination gives a different trait. Let us look at an imaginary gene which can have the alleles R, R', r and r'. Let's say this gene of my father's has the alleles R and R', and my mother's has the alleles r and r', then their alleles of the gene are RR' and rr' respectively.

    Since their son, me, inherited exactly one allele from each parent, i could have either Rr, Rr', R'r or R'r'. On the other hand, my brother have also inherited one of the four combinations of alleles. As a result, we could have inherited the same alleles, e.g. Rr and Rr, half of the same alleles, e.g. Rr and Rr', or we could have inherited completely different alleles, e.g. Rr and R'r'.

    Likewise, the same rule applies to all the other genes of my brother and me.

    The relatedness tutorial says, "As a result, siblings could share no alleles (in the unlikely case that each child got the half of the genes from each parent that the other didn’t), all alleles (in the unlikely case that both children got the same set of genes from each parent), or anywhere in between."

    Although "siblings share half their alleles on average", it's only an average and can vary significantly, from 0% to 100% of all alleles. On the other hand, first cousins share 25% of their alleles on average and varies from 0% to 50%.

    Even though on average, siblings share more alleles than cousins do, there is clearly a slight chance (significant enough) that cousins can share more alleles than siblings do.

    Please take a look at the link provided and comment.
  7. Sep 24, 2013 #6


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    Some cousins can share more alleles than other siblings might share - they cannot share more alleles than their direct parents do, if the relationship between their other parents and that family is negligible (=> no incest).
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