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7.2.2Genetics, Populations, Evolution and Ecosystems (A Level only)

Gene Pools and Allele Frequency — AQA A Level Biology 7402 Revision Notes & Topical Questions

5 min read 9 parts Lesson 7 of 25 1 quiz

📚 AQA A Level Biology 7402 Hub / Topical Revision
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7.2.2 Gene Pools and Allele Frequency — AQA A Level Biology 7402 notes: Calculate allele frequencies from genotype or phenotype data and explain how selection and other factors alter them.The gene pool of a population contains every allele of every gene.For a gene with two allele…

  • Natural selection
  • Genetic drift
  • Gene flow (migration)
  • Non-random mating
  • phenotypes
  • balanced polymorphism

Section 1 of 9Allele frequency in a gene pool

The gene pool of a population contains every allele of every gene.

Allele frequency
For a gene with two alleles, the dominant allele A and the recessive allele a, the allele frequency is the number of copies of that allele divided by the total number of copies of the gene.
P + q = 1
Frequencies are written as decimals: the frequency of A is p and of a is q, and p + q = 1.

Remember that each diploid individual carries two copies of each gene, so a population of N individuals has 2N copies of the gene.

Section 2 of 9Calculating from genotype numbers

If the numbers of AA, Aa and aa individuals are known, count the alleles directly. Each AA individual contributes two A alleles, each Aa contributes one A and one a, each aa contributes two a alleles.

Worked pattern

  • in 100 individuals there are 36 AA, 48 Aa and 16 aa.
  • Alleles = 200.
  • A = (2 x 36) + 48 = 120, so p = 120/200 = 0.6. a = (2 x 16) + 48 = 80, so q = 0.4.
  • Check: 0.6 + 0.4 = 1.

From a phenotype alone you can only count the homozygous recessive directly (because their genotype is known). Frequency of aa = q2, which is the basis of the Hardy-Weinberg equation (7.2.3).

Section 3 of 9Why allele frequencies change

Allele frequencies in a real population are rarely constant. They change through:

1Natural selection
Natural selection alleles that improve survival and reproduction become more frequent; harmful ones become rarer.
2Mutation
Mutation introduces new alleles at low rates; the effect on frequency is small in a single generation.
3Genetic drift
Genetic drift random changes in allele frequency by chance, strongest in small populations. An allele can be lost or become fixed purely by chance.
4Gene flow (migration)
Gene flow (migration) movement of individuals and their alleles between populations alters frequencies in both.
5Non-random mating
Non-random mating e.g. sexual selection or inbreeding changes genotype frequencies.

Section 4 of 9Natural selection and the shift in frequency

Take a gene controlling a feature that affects survival.

Natural selection and the shift in frequency
In an environment where the dominant allele gives a selective advantage, individuals with genotype AA and Aa survive and breed more than aa.
Over generations, q falls and p rises.
Phenotypes
Selection acts on phenotypes, but the outcome is a change in allele frequency.
If the recessive phenotype is lethal or strongly disadvantageous, the recessive allele persists in heterozygous carriers, who are not eliminated, so q falls very slowly.

Example

  • antibiotic resistance.
  • Before antibiotic use, resistance alleles are rare.
  • When an antibiotic is applied, only resistant bacteria survive and reproduce, so the resistance allele frequency rises sharply;
    • this can happen rapidly because bacteria reproduce quickly and can also pass alleles by horizontal transfer of plasmids.

Section 5 of 9Interpreting data

When asked to describe a graph or table of allele frequency over generations, state the overall trend, quote values with units and time interval, and identify any rapid change.

Interpreting data
Link the change to the selection pressure in the question (e.g. a pesticide, predator or change in climate).
A gradual rise in allele frequency suggests selection of a modestly advantageous allele; a sudden random shift in a small population suggests drift or a founder effect.
FactorEffect on allele frequency
SelectionDirectional change favouring advantageous allele
DriftRandom; large effect in small populations
Gene flowMakes populations more similar
MutationAdds new alleles slowly

Section 6 of 9Worked examples with selection

Suppose a recessive allele a causes a mild disadvantage and the frequency of a falls from 0.30 to 0.25 in one generation. You can describe this as a decrease of 0.05, or as a percentage decrease: (0.05 / 0.30) x 100 = 16.7%. Be clear which one the question asks for.

Consider also the sickle cell allele.

In regions where malaria is common, heterozygous individuals (HbAHbS) have an advantage because they are more resistant to malaria than HbAHbA and do not suffer the severe anaemia of HbSHbS.
Balanced polymorphism
Selection therefore maintains the HbS allele at a high frequency (a balanced polymorphism) even though homozygous HbSHbS is harmful.
Where malaria is absent, the advantage disappears and the HbS allele frequency falls over generations.
This shows that whether an allele is advantageous depends on the environment, and that selection acts on phenotype, with allele frequency as the outcome.

Always finish a longer answer by stating the consequence for the gene pool: the proportion of copies of the allele in the population has changed, and if the change continues over many generations the population has evolved.

Section 7 of 9Exam-style questions

  1. In a population of 200 plants, 50 are RR, 100 are Rr and 50 are rr. Calculate the frequency of the allele R. [3]3 marks

  2. A herbicide is sprayed on a field of weeds each year. The frequency of an allele for resistance rises from 0.02 to 0.45 over 10 years. Explain this change. [4]4 marks

  3. Explain why genetic drift has a greater effect on allele frequency in a small population than in a large one. [2]2 marks

Section 8 of 9Key terms

TermMeaning
Allele frequencyProportion of all copies of a gene in the gene pool that are a particular allele.
Gene poolTotal alleles of all genes in a population.
Genetic driftRandom change in allele frequency due to chance events, most important in small populations.
Gene flowMovement of alleles between populations by migration and breeding.
Selection pressureAn environmental factor that affects survival and reproduction of phenotypes.
Founder effectReduced diversity when a small group starts a new population.

Section 9 of 9Exam technique

1
Forgetting to double the number of individuals to count alleles (diploid organisms have two copies).
2
Giving allele frequency as a percentage when asked for a decimal, or vice versa.
3
Writing that selection changes the allele itself; it changes the frequency of the allele.
4
Assuming a harmful recessive allele disappears quickly; heterozygotes carry it unseen.
5
Not linking the change in frequency to a named selection pressure in the question.

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