Human inbreeding: the logic behind it

Earlier in this blog I have written twice about human inbreeding. I never thought that my writing would attract so much attention. The first article, a compilation of a few readily available scientific papers and possibly a few articles in Google, was my effortless entry to the blogosphere. The attention I received inspired me to delve into this issue even more which brought about the second article, which comprised of considerable archeological excavation of scientific literature. I appreciate all the comments that readers have provided. I can only hope, whether you agree or disagree with me, that my articles have furthered your understanding of human inbreeding.

Many of the readers who cared to express their disagreement with me mentioned that even though the entire royals of UK, the Japanese Monarchs, and the ancient Egyptian royals are products of inbreeding, they do not show any aberrations. In case of British Royals, hemophilia may not have been due to inbreeding as another author pointed out. Yet another reader giving her own example said that despite being inbred, her brother and she are fine “just like everybody else.” I completely agree with these arguments and arguments like these that just because one is descended from a consanguineous marriage, one may not show any aberrations. However, one should always consider that there may be recessive lethal mutations in us and the probability of such mutations being passed on is highly increased in consanguineous marriage.

For example, assume there is a gene P that is vital for life. In any population, because of mutations there can be various versions of gene P. Let us assume that there are three functional versions P, P’, and P.” The fourth version p let us assume is deleterious and recessive. All four alleles are distributed equally. From this description, individuals with genotype PP, PP’, PP”, P’P” are fully functional whereas Pp, P’p, P”p are carriers and pp individuals show symptoms. In this population, the allele frequency for p is 1/4 and the chances of one being pp is 1/10. This means that the probability of a person showing the mutation is 10%. Now let us assume that in this population there is a family in which a son (or a daughter) has the recessive deleterious allele p such that his genotype is –p (it does not matter what his other allele is. Thus, the functional alleles are indicated by a dash). Assuming his wife had two of the functional alleles –, the chances of his children inheriting the allele p is 1/2 (in the population the risk was 1/4) and the chances of pp individuals are 0. Let us assume he had for children with the following genotype:–, -p, –, -p. If the kids with –p genotype marry with people of — genotype, then the chances of
-p individuals in next generation is again 1/4 and the same for pp 0 but if they get married together the chances of –p is 1/2 and now the probability of pp individuals is 1/4 which is 25% that is 15% higher than the general population. This is where inbreeding matters.

Now having that said, there are a few things that we need to consider before discussing the effects of inbreeding. First, only the mutations that can be seriously detrimental can be easily reported and measured. If one is a carrier (-p genotype) it becomes harder to characterize them. Second, if the mutations are in the genes that are important but not life threatening, the individuals carrying the mutations may appear normal. These mutations include genes responsible for eye colors and hair colors. Third, some fetuses carrying detrimental mutations can result in miscarriages and stillborn babies and those that live may only last for a few years. We only know about the mutations in famous people and those in general public may not be known. Therefore, although the royals may be inbred yet okay, we cannot assume that inbreeding is okay.



Filed under Evolution, Human Evolution

6 responses to “Human inbreeding: the logic behind it

  1. S Weitze

    Hey Aashish,

    Funny that I happened across your blog, glad the class was able to inspire your thoughts and reading!

    Regarding human inbreeding, thought you’d find this recent article interesting:

    Hope you are well!

  2. Professor…I know you don’t like it…but thanks for taking some time to jot down the encouraging words. The class was hard but great. I read the report and the original article in Plos Biology. The paper is a good and comprehensive summary.

    The original paper freely available at Plos Biology mentions that “even assuming that the deleterious phenotype arises solely from homozygosity at a single locus, the increased risk depends on the frequency of the allele involved; it is not an immediate consequence of the degree of relatedness between cousins.” This kind of surprises me though. The paper also points out that “children of cousin marriages are likely to manifest an increased frequency of birth defects showing polygenic inheritance and interacting with environmental variation” which I have also concurred with earlier.
    The authors also mention that the genetic risks of inbreeding can only be accessed “empirically” in population-level and those risks can not be generalized as they are “specific to particular populations in specific environments.”

    Extensively inbreed dogs demonstrate serious physical defects. Then there are pure bred mice. There were also reports about declining lion population due to susceptibility to diseases because of decreasing genetic diversity. There are other reports on negative consequences of inbreeding in animals but there are not that many in humans (to my knowledge). I read Mutants by A.M. Leroi and the rare mutations that have been described in the book that are even more convincing. I can only hope that the readers will read both those books and the Plos article. I also hope that more scientific work will be done in near future in this area.

    Happy new year 😀

  3. According to Labouriau and Amorim in “An Association Between the Kinship and Fertility of Human Couples” published in Science (12 Dec 2008, Vol. 322. no. 5908, p. 1634)
    “[H]uman fertility often increases with kinship until a maximum and then decays as the kinship increases further……… Although the results discussed here are expected from the theory of inbreeding and outbreeding depression, they are hard to document in human populations. Reliable, sufficiently large human data sets, as in the two studies discussed here, are uncommon. Moreover, it is difficult to eliminate the possibility of spurious associations due to effects of socioeconomic factors unequivocally. A future challenge will be to discern which of the many possible mechanisms underlying inbreeding and outbreeding depression are in play.”

  4. Just another artifact to prove my point about the lack of scientific research in the area of human inbreeding:

  5. Pingback: Human Inbreeding: A better understanding « Mentavolution

  6. I have long suspected that the costliest and so-called common diseases were due to inbreeding and puely Preventable.
    The trick is to look up each disease separately and you’ll find the data that most if not all of the costliest and most common (also the more exotic ones) diseases are preventable.

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