A-Level · Biology · AQA · Mark scheme decoded
AQA A-Level Biology: Population Genetics and Hardy-Weinberg Principle — mark scheme explained
The short answer
In the study of genetics, populations, evolution, and ecosystems, understanding how genetic variation is maintained within a population is crucial. This section focuses on the concepts of species, populations, gene pools, allele frequencies, and the Hardy-Weinberg principle. We will explore these ideas in detail to provide a comprehensive understanding of population genetics.
The question
In a population of 100 individuals, 36 are homozygous dominant (AA), 48 are heterozygous (Aa), and 16 are homozygous recessive (aa). Calculate the allele frequencies p and q. [Paraphrased for study — not reproduced from any exam paper.]
Mark scheme, decoded
What each mark is really for — in plain English — and the wording trap that loses it.
- S1
Step 1: Determine the total number of alleles in the population.
- S2
Total alleles = 2 × 100 = 200
- S3
Step 2: Count the number of each allele.
- S4
Number of A alleles = (36 × 2) + 48 = 72 + 48 = 120
- S5
Number of a alleles = (16 × 2) + 48 = 32 + 48 = 80
- S6
Step 3: Calculate the frequency of each allele.
- S7
p = 120 / 200 = 0.6
- S8
q = 80 / 200 = 0.4
Model answer
Worked through, with each step tagged to the mark it earns.
- S1
Step 1: Determine the total number of alleles in the population.
- S2
Total alleles = 2 × 100 = 200
- S3
Step 2: Count the number of each allele.
- S4
Number of A alleles = (36 × 2) + 48 = 72 + 48 = 120
- S5
Number of a alleles = (16 × 2) + 48 = 32 + 48 = 80
- S6
Step 3: Calculate the frequency of each allele.
- S7
p = 120 / 200 = 0.6
- S8
q = 80 / 200 = 0.4
Final answer: p = 0.6, q = 0.4
Common mistakes
- Confusing the total number of alleles with the total number of individuals. — Always remember to multiply the number of individuals by 2 to get the total number of alleles.
- Forgetting to use p + q = 1 when calculating allele frequencies. — Always check that the sum of p and q equals 1 after calculating each frequency.
- Incorrectly applying the Hardy-Weinberg equation. — Practice using the Hardy-Weinberg equation step-by-step. Ensure that p 2 , 2pq, and q 2 are calculated correctly and sum to 1.
- Misinterpreting the conditions for Hardy-Weinberg equilibrium. — Review and memorize the five conditions required for Hardy-Weinberg equilibrium. Practice identifying which conditions are met or not met in given scenarios.
- Failing to check if the sum of genotype frequencies equals 1. — Always double-check your calculations by ensuring that the sum of genotype frequencies equals 1.
- Incorrectly interpreting allele and genotype frequencies in a population. — Practice converting between allele and genotype frequencies using the Hardy-Weinberg equation. Understand that p 2 , 2pq, and q 2 represent specific genotypes in a population.
Where the marks go
- Full worked solution (all marking points)3 marks