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Destroyer Bio Q.

Discussion in 'DAT Discussions' started by Golfguy, Jul 24, 2011.

  1. Golfguy

    5+ Year Member

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    The frequency for the allele for cystic fibrosis, an autosomal recessive trait is 1 in 2500 among white Americans. What is the heterozygote frequency?

    A. 8%
    B. 4%
    C. 1%
    D. 10%
    E. 14%

    The answer is 4%. What I don't understand is why q^2= .0004 (1/2500). Shouldn't q =.0004? I thought q was the frequency of the recessive allele, and q^2 was the phenotypic frequency. Can someone please explain this to me?

    Thanks Much!
     
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  3. PooyaH

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    When it says it's an autosomal RECESSIVE trait, it's talking about the genotype. And since CF is a recessive disorder, you have to be homozygous recessive to show it. But I think it's not a very well-worded problem. It shouldn't say the frequency of the "allele" in the beginning of the question. That's def misleading.
     
  4. Hope30

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    Regarding the question, I tried to solve it and have something close to 4%, but not sure it is right.

    p^2= 1/2500 --> take the square root of 1/2500 will give you p=0.02
    p + q = 1
    1 - 0.02 = 0.98

    p^2 + 2pq + q^2 = 1
    The question asks for heterozygous, so use 2pq
    2(0.98)(0.02) = 0.0392
    0.0392*100% = 3.92% close to 4%
     
  5. Golfguy

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    Thanks guys. It seems as if it's just worded incorrectly then. It should read the prevalence of cystic fibrosis is......not allele prevalence.

    I thought the frequency of an allele is p or q, not p^2 or q^2.
    I thought the frequency of the phenotype is p^2, or q^2.

    For example, if 80% of a population express the dominant phenotype, then p^2 + 2pq = .8, and q^2 = .2 for the recessive.

    So, solving from the recessive (easier). We would get q = .44, 1-.44= .56 = p.

    Does this look correct?

    Oh, and Hope, you are correct.
     
  6. PooyaH

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    Yes you are correct. q and p refer to the frequency of the alleles. p^2, 2pq, and q^2 refer to the frequency of genotypes among a population.
     
  7. Golfguy

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    Thanks PooyaH!
     

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