Biology - Frequency of allele problem

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Discussion Overview

The discussion revolves around a homework problem related to calculating allele frequencies using Hardy-Weinberg's Law, specifically focusing on an autosomal dominant trait related to smell hypersensitivity to asparagus. Participants explore the methodology for determining allele frequencies based on given genotype data.

Discussion Character

  • Homework-related
  • Mathematical reasoning

Main Points Raised

  • One participant outlines their approach to solving the problem, defining the alleles as 'A' for the dominant trait and 'a' for the recessive trait, and calculates the frequency of the recessive genotype (aa) as 16 out of 35 individuals.
  • Another participant suggests that the initial calculations appear correct but hints that the discrepancy in the total number of individuals may stem from rounding errors.
  • A participant expresses uncertainty about whether to round the calculated value of 2pq (5.3) to 5 or 6 individuals, indicating a concern about how rounding affects the final tally.
  • Further clarification is provided regarding the implications of rounding, noting that rounding down may lead to a loss of precision in the total count of individuals.
  • Another participant discusses the next steps in the calculation, indicating the need to determine the frequency of the dominant allele based on the adjusted number of individuals.

Areas of Agreement / Disagreement

Participants generally agree on the approach to the problem but express differing views on how to handle rounding and its impact on the final calculations. The discussion remains unresolved regarding the best method for rounding and its implications for the results.

Contextual Notes

Participants acknowledge potential limitations related to rounding errors and the impact of these errors on the total count of individuals in the population, which may affect subsequent calculations.

v_phoenix
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Hello,

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


Homework Statement

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!
v_phoenix
 
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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.
 
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??
 
v_phoenix said:
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??

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.
 
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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