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Biology · University

Population genetics and model assumptions

Allele frequency counts copies of an allele, not just individuals carrying it. Diploid genotype counts require two allele copies per individual.

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Prerequisites

Relevant school subject foundations and careful reasoning.

Study notes

Alleles

Allele frequency counts copies of an allele, not just individuals carrying it. Diploid genotype counts require two allele copies per individual.

Equilibrium model

For two alleles with frequencies p and q, p+q=1. Hardy-Weinberg genotype proportions p²,2pq,q² require specific population-model assumptions.

Processes

Selection, drift, migration and mutation can alter allele frequencies. A small sample also introduces sampling variation.

Inference

An observed deviation from a simple model does not alone identify its cause. Consider sampling and alternative biological processes.

Worked examples

A sample contains 10 AA,20 Aa,10 aa diploid individuals. Find frequency of A.

(2×10+20)/(2×40)=0.5.

0.5

For p=0.6 in a two-allele model, find q.

p+q=1.

0.4

Check your understanding

  1. Under Hardy-Weinberg assumptions with p=0.6,q=0.4, find expected heterozygote proportion.

  2. Which counts allele frequency correctly?

    1. Only the largest genotype group
    2. Only individuals with at least one copy
    3. Only dominant phenotypes
    4. Allele copies divided by all relevant copies

  3. Why are Hardy-Weinberg proportions not a universal rule for every observed sample?

  4. Which process can change allele frequencies by random sampling?

    1. Genetic drift
    2. A change in chart font
    3. Changing units only
    4. Relabelling identical data

Answers and marking guidance

Answers and marking notes
  1. 0.48

    2pq=2×0.6×0.4.

  2. Allele copies divided by all relevant copies

    Diploids can carry zero, one or two copies of the allele.

  3. They follow specified assumptions and refer to a population model; sampling variation and biological processes can produce departures.

    Credit model assumptions and finite samples.

  4. Genetic drift

    Drift refers to random changes in allele frequencies.

Further reading and review references

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