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Is it true? about born handicap

  1. Dec 17, 2009 #1
    Hello, Chitose wonder chick again.

    It's about incest.
    I don't care about it's illegal or not, I just want to know if it's true or not?

    That if people have kid within own family, chance of that kid become handicap are very high
    (Like blood relate brother and sister haveing a kid)

    IS that really true?
    If it's true, Why? a chromosome thing?

    I ask because there's some case happan to people I know is most likely these. They are real brother and sister but sister have sterile, so they marry to themself and prepared to abdopted child as their son.

    And... Don't take this seriously, People that belive in Adam and Eve, wich mean they belive in incest is common thing for human too?

    PS. English is not my native language, Please forgive If i'm worng in gramma or spelling.
  2. jcsd
  3. Dec 18, 2009 #2
    Chances of handicap are not very high, but a lot higher than if parents are not related.

    Many people have malfunctions of some sort in their genetic code. These malfunctions do not show up as disorders, because most genes come in two copies (one from the father and one from the mother) and you need exactly the same malfunction in both copies of the gene to result in the disorder.

    If parents are not related, it's very unlikely that they'll have the same mutation. If they are brother and sister, there's a good chance that they've inherited the same mutation from one of their parents. If that's the case, there's a chance that their children will have two bad copies of the gene.

    Does that make sense?
  4. Dec 18, 2009 #3


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    Typically, really serious "malfunctions", like mongoloidism (apologies to all our Mongolian friends for that term!) or hemophilia, are recessive genes- that's because if a serious malfunction were dominant, people carrying that gene would all die out and the gene would disappear from the gene pool. Closely related people are much more likely to be carrying the same genes, so the same recessive genes, so the same "bad" genes.

    "In breeding" is why the ruling families of Europe had such a high percentage of hemophiliacs- not to mention the famous "Hapsburg Lip" (which was actually a very pronounce "prognathis" (buck teeth)).
  5. Dec 18, 2009 #4


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    It entirely depends on the degree of relatedness. If the persons are directly related (parent/offspring or siblings) the chances of a handicap are high, they should not be having children. The odds are much lower and acceptable for niece/nephew couples.

    That's true in the case of a recessive mutations.

    The PC-term is Down syndrome. It is a trisomy of chromosome 21, so it is not correct to say that it is a recessive gene.
  6. Dec 18, 2009 #5
    Assuming brother/sister incest: for any given recessive mutation, chances of a handicap are equal to the chance that one of the common parents has the recessive gene (for example, for phenylketonuria that's 1 in 25), times the chance that both the brother and sister get the bad gene (1 in 4), times the chance that their child gets two bad genes (1 in 4), so, for phenylketonuria the odds are 1 in 400, which is fairly low, but much higher than odds of the same disorder for the child of two unrelated people (1 in 10,000).

    Hemophilia is not a very good example, because it's a X chromosome recessive disorder and things work out differently in that case.
  7. Dec 18, 2009 #6
    I see... so, even normal people have bad gene inside but it's not show effect because it's not in pair right? people for same family share same blood and most likely same bad gene, if they have a kid, bingo thier kid will recive bad gene in pair and become handicap.

    but... how far blood line is safe?

    you know, even two people from different place and parant, they great great great great grandfather migh in same family. the world is big anything like can be happen.
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