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What is the probability that the child is affected?

  1. Dec 8, 2018 #1
    1. The problem statement, all variables and given/known data
    The following pedigree shows that the trait is autosomal recessive. The probability of carriers in the population is 1/3. What is the probability that the child of III-3 and 4 is affected?
    2. Relevant info
    The answer provided is 1/36.

    3. The attempt at a solution
    The probability of II-3 being a carrier is 1/2 and that of II-4 is 1/3. The probability of two carriers having an affected child is 1/2. So the probability that II-3 and II-4 are carriers and their child is affected is 1/2 x1/3 x1/2=1/12.

    (Edited to remove typo.)
    Last edited: Dec 8, 2018
  2. jcsd
  3. Dec 8, 2018 #2


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    Hm. There seems to be a problem with II versus III in your solution.
    II is the middle gen. III is the last gen.

    What is the probability of II3's inheritance from one I1? What is the probability from I2?
    What is the total?
  4. Dec 8, 2018 #3
    The probability of inheritance of a mutant allele is 1/2 from I-1 and 1 from I-2. The probability of II-3 being a carrier is 1/2.
  5. Dec 8, 2018 #4


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    OK, but one carrier and one not carrier don't have the same odds as two carriers.
    Hint: what does it mean that the gene is recessive?
    How do recessive genes manifest?
  6. Dec 10, 2018 #5
    Recessive alleles manifest only when they occur in a pair. To be honest I'm not able to follow your indication.
    Last edited: Dec 10, 2018
  7. Dec 10, 2018 #6


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    This is incorrect. It may be helpful to try writing out the genotypes of the parents (I1 and I2), then trying to infer what the genotype of II3 must be.

    This is incorrect. Try drawing a Punnet square of the cross.

    When I analyze the problem, I do not get this answer.
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