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Old 05-02-2011
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Genetics Genetics consanguinity

this is from Genetics Kaplan lectures notes.

A man who is a known heterozygous carrier of oculocutaneous albinism marries his half cousin (they share one common grandparent). This trait is transmitted as a fully penetrant autosomal recessive. What is the probability that this couple will produce a child with this disorder?

The answer is 1/64.

but I have a question.

If they are not relatives , and in the same case (both heterozygous) the probability that this couple produce a child with that disorder will be 1/4 right?


So it isn't supposed that consanguinity increase the risk of have the disease?

Does anyone coulp explain me this concept plz?
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Old 05-02-2011
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So, they are not necessarily both heterozygous... Our proband is confirmed heterozygous, but we don't know which parent he got it from or which parent that parent got it from. The chances are:
  • 1/2 that he got it from his mom, then
  • 1/2 that she got it from her dad, then (back down the other side of the family tree)
  • 1/2 that he gave it to his child by another wife, then
  • 1/2 that that child gave it to his daughter (proband's half-cousin & wife), then
  • 1/2 that she will pass on a defective allele, and
  • 1/2 that proband will pass on his defective allele.
Multiplying all the probabilities, we get (1/2)^6 = 1/64
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Old 05-03-2011
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yes I agree with that but my question is : if they are cousins like the case the probability is 1/64 but if they weren't relatives the probability will be 1/4 right? let me know if I am wrong. so that mean that consanguinity reduces the probability of disease? How is that? it have no sense for me.
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Old 05-03-2011
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Quote:
Originally Posted by acr2503 View Post
yes I agree with that but my question is : if they are cousins like the case the probability is 1/64 but if they weren't relatives the probability will be 1/4 right? let me know if I am wrong. so that mean that consanguinity reduces the probability of disease? How is that? it have no sense for me.
If they were not cousins, the probability of each one of them being a carrier (heterozygous) would be similar to the population incidence of heterozygotes which is definitely less than 1/64, it can be 1/10,000 for example. So their probability of producing an affected child will be something like 1/10,000 X 1/10,000 X 1/4
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Old 05-06-2011
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ok I got it thx!!
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Old 05-06-2011
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Ohhh woww! Thanksss for explaining
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