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Incest: Why does it cause defects?

  1. Jan 5, 2010 #1
    Why does incest cause defects in organisms? If humans have sex with other humans that are related the baby will most likely have a defect. But If you have sex with someone who you are not so closely related to your chances of having a defect are less. Why is that? Humans had to have started out sometime in the past, right? How did they reproduce without having defects. We know that population increases (in our world today) so it had to have had a beginning. How did the earliest people reproduce with each other if there wasn't a lot of them to choose from like today?

    I made an identical thread. Post replies in this one. I didn't mean to.
    Last edited: Jan 5, 2010
  2. jcsd
  3. Jan 5, 2010 #2


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    Humans didn't "start out" in the Adam and Eve sense of the phrase. All living things share a common descent.

    Genetic diversity is important because it makes sure a population doesn't have any one thing that will totally wipe it out.

    It also has to do with genes having dominant and recessive alleles.

    Recessive alleles are expressed (or rather, not masked) if the person does not have a dominant allele. This means a person with a 'bad' recessive allele paired with a 'good' dominant allele will be selected for based upon the good allele and the recessive allele will still have a chance of getting passed around.

    The closer you are related to somebody the higher the chance that you both carry the same copy of a recessive allele and so the likelihood of a potentially bad allele getting expressed in their offspring is higher.
    Last edited: Jan 5, 2010
  4. Jan 5, 2010 #3


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    Welcome to PF!

    Hi Fortifiv2! Welcome to PF! :smile:
    If a closely related male and female ("the first generation") were stranded alone today on an island, with no contact with anyone else for a thousand years, and if they bred, then the second generation would be likely to have a high proportion of defects.

    If the second generation bred randomly, then the third generation would have an equally high proportion of defects (or is it higher? :confused:).

    But (survival of the fittest), the less fit will probably have fewer children, and the more fit will be more likely to breed with each other than with the less fit. So the proportion of defects would be less.

    In each succeeding generation, for the same reasons, the population would separate out, with the more defective either dying out or living, and interbreeding, in their own communities. And the more fit would look at them, and would avoid breeding with close relatives.

    After a thousand years, most of the population would be free of the original defects (though of course, new mutations would still be expected), with only small backward groups marrying close relatives.

    By comparison, when "the first humans" arrived, they were a defect, but a "good defect"!

    Breeding with close relatives is an advantage for any defect, and is therefore an advantage for the species if it's a "good defect"!

    At some stage, the advantage of preserving the "good defect" by inbreeding would be countered by an increasing proportion of "bad defects", and a tendency not to inbreed would have become "fittest for survival." :smile:
  5. Jan 5, 2010 #4

    Andy Resnick

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  6. Jan 5, 2010 #5
    "Consanguineous [cousin] unions remain preferential in North Africa, the Middle East and large parts of Asia, with marriage between first cousins being particularly common." (Consanguinity and child health, Anand K. Saggara and Alan H Bittles)

    "In many cases incest is also inbreeding. Inbreeding leads to a higher proportion of congenital birth defects through an increase in the frequency of homozygotes." (http://en.wikipedia.org/wiki/Incest#Inbreeding")

    While inbreeding results in a higher proportion of defectives of children born to closely related parents, over time, it also works to purify the blood line, particularly when the defectives are culled. Obviously, those who die as a result of their defects are culled. Many historical cultures, however, examined babies and performed culling manually.

    Indeed, inbreeding was used for the most common laboratory rat, the albino Wistar rat. All members of this strain are genetically uniform.
    Last edited by a moderator: Apr 24, 2017
  7. Jan 5, 2010 #6
    The Illinois Annotated Statues describes how while revising the state's laws in the late sixties, the legislature asked geneticists to testify regarding whether there were genetic reasons why incest should be illegal. Their testimony was that in order for incest to result in increased birth defects, the defective genes have to be both rare and recessive. If they aren't rare the birth defect is as likely to occur in unrelated couples. If not recessive, only one partner need have the defective gene.

    They also testified that any birth defect tends to reduce that person's chance of having offspring thus reducing the occurrence of that defective gene in the population. They concluded there was no genetic reason for prohibiting incest (among consenting adults). Nevertheless the legislature continued the prohibition.
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