Hardy weinberg
Laura Armstrong & Joe Wolfensohn
Teachers
Contents
Recall Questions
This topic requires prior knowledge of key inheritance terms. You can test your knowledge on these below.
What is an allele?
A different version of the same gene.
What does it mean if an organism is homozygous for a gene?
It has two identical alleles for that gene.
What causes allele frequencies to change over time?
Mutation, natural selection, genetic drift, migration.
Topic Explainer Video
Check out this @JoeDoesBiology video that explains hardy weinberg or read the full notes below. Once you've gone through the whole note, try out the practice questions!
What is the Hardy-Weinberg Principle?
The Hardy-Weinberg Principle is a mathematical model used to estimate the frequency of alleles and genotypes in a population, and to determine whether evolution is occurring.
It predicts that allele frequencies in a population will remain constant from one generation to the next if no evolutionary forces act on the population.
Hardy-Weinberg Equations
There are two essential equations:
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Allele Frequency Equation:
- p + q = 1
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p = frequency of the dominant allele
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q = frequency of the recessive allele
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Genotype Frequency Equation:
- p2 + 2pq + q2 = 1
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p² = frequency of homozygous dominant genotype (e.g., AA)
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2pq = frequency of heterozygous genotype (e.g., Aa)
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q² = frequency of homozygous recessive genotype (e.g., aa)
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What Is It Used For?
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To estimate allele frequencies and genotype frequencies in populations.
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To determine whether a population is evolving (if actual genotype frequencies deviate from expected).
- To track the inheritance of recessive diseases in populations (e.g., cystic fibrosis, sickle cell anaemia).
Practice Questions
In a population of mice, the frequency of the recessive allele for white fur (a) is 0.2. Use the Hardy-Weinberg principle to calculate the expected percentage of mice that are heterozygous. (3 marks)
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q = 0.2 (frequency of allele a)
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So, p = 1 − 0.2 = 0.8 (frequency of allele A)
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Heterozygous frequency = 2pq = 2 × 0.8 × 0.2 = 0.32
- As a percentage = 32%
The Hardy-Weinberg equation can be used to estimate the frequency of a recessive allele in a population. Haemochromatosis is a condition caused by a recessive allele. In one country, 1 in every 400 people was found to have haemochromatosis.
Calculate the frequency of people who are healthy but carriers (heterozygotes) of the allele for haemochromatosis (3 marks)
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1/400 people have the condition
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This condition is caused by a recessive allele so they must be homozygous recessive.
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Therefore q2 = 1/400
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q2 = 0.0025
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q= √0.0025 = 0.05
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p = 1 – 0.05 = 0.95
- 2pq = 2 x 0.95 x 0.05 = 0.095
Hardy-Weinberg Assumptions
For the equations to be valid, the following conditions must be met:
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No mutations occur.
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No natural selection is acting on the alleles.
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The population is large (to avoid genetic drift).
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Random mating occurs.
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No immigration or emigration (closed population — no gene flow).
If these conditions are not met, allele frequencies will change — indicating evolution is occurring.
Key Terms
- Allele: A different version of a gene.
- Allele Frequency: The proportion of a specific allele among all alleles in a population.
- Genotype Frequency: The proportion of individuals with a specific genotype.
- Hardy-Weinberg Principle: A model that predicts allele/genotype frequencies in a non-evolving population.
- Genetic Equilibrium: A state where allele frequencies remain constant over generations.
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Exam Tips
Setting out your working logically and clearly is crucial!
1. Write down what you are trying to work out: It is often the frequency of the heterozygous genotype (2pq).
2. Write down what you know: They will have given you information in the question, for example, they may have told you the frequency of the recessive allele (q) or the homozygous recessive genotype (q2).
3. Use what you know to calculate the answer: For example, once you know p, you can calculate q then you can calculate 2pq.
Don’t forget that frequencies are proportions — always between 0 and 1. If asked for percentages, multiply by 100.
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Complete achromatopsia is a form of complete colour blindness. People with complete achromatopsia have difficulty in seeing detail. Complete achromatopsia is caused by an autosomal recessive allele.
Ten percent of the population on the Pacific island of Pingelap are affected by complete achromatopsia.
Use the Hardy-Weinberg equation to calculate the percentage of this population who are heterozygous for this disorder. (2 marks)
10% of the population have complete achromatopsia.
These individuals must be homozygous recessive.
So q2 = 0.10
So q = √0.10 = 0.32
So p = 1 – 0.32 = 0.68
So 2pq = 2 x 0.68 x 0.32 = 0.44
0.44 x 100 = 44%
Practice Question 1
Try to answer the practice question from the TikTok on your own, then watch the video to see how well you did!
Practice Question 2
Try to answer the practice question from the TikTok on your own, then watch the video to see how well you did!