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Biology - Frequency of allele problem

  1. Apr 15, 2008 #1

    I've been given this problem involving allelic frequencies and I used the Hardy Weinberg's Law as an attempt to solve it

    The problem statement, all variables and given/known data

    In a sample of 35 people, 19 have specific smell hypersensitvity to asparagus. Calculate the frequency of the allele in this sample given that the trait is autosomal dominant.

    The attempt at a solution

    Here is my working...
    I represented the autosomal dominant faulty gene by 'A' and the working copy of the gene by 'a'.

    AA = ?
    Aa = ?
    aa = 16 <<< # genotype (aa): 35-19 = 16
    Total 35

    I knew that the frequency of non affected recessive individuals = 1/35

    Hence q^2 = 1/35... thus q=0.169 (3dp)

    Hence p = 1 - 0.169 = 0.831 (3dp)...it then follows p^2 = 0.691 (3dp)

    So putting it all together i got... 2pq = 2 * 0.831 * 0.169 = 0.28059

    So after that I thought we should multiply these frequencies by the 19 people before to calculate the individual genotype frequencies...

    AA = 13 <<< (p^2*19)
    Aa = 5 <<< (2pq*19)
    aa = 16
    Total = 34

    But here my total adds up to only 34 people!!
    I'm not sure if i'm doing it right or i'm missing something...

    Please help, any suggestions will be greatly appreciated!!
  2. jcsd
  3. Apr 15, 2008 #2


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    I haven't checked through it carefully to check for errors, but a quick scan looks like you're doing it right. I think your problem in checking your answer at the end (always a good strategy) is due to rounding error.
  4. Apr 15, 2008 #3
    Oh yes, I did check my rounding. But I wasn't too sure when I got 2pq=5.3... if i should keep it at 5 people or round to 6 people??
  5. Apr 15, 2008 #4


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    When I said error, I didn't mean a "mistake," but error in terms of the uncertainty added by rounding. 5.3 would round to 5, but that means you've discarded 0.3. It's not wrong to do that, but means the numbers might not add up perfectly when converted to whole numbers.
  6. Apr 15, 2008 #5
    hmmm... so do you mean i should keep it as 5 people with a genotype of Aa??

    Because from there i need to do another step... where I multiply the number of people with the copy of big A allele and divide by the total number of genes in the population.

    So if i use 34 people it gives me a result of... ((2*13) + 5) / (2*34) = 31/68
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