A group of organisms of the same species occupying a particular space at a particular time that can potentially interbreed.

All the alleles of all the genes in a population at a given time.

What is allele frequency?

The proportion of all copies of a gene in a population that are a particular allele (a value between 0 and 1).

What does the Hardy-Weinberg principle predict?

That allele frequencies stay constant from generation to generation when its conditions apply.

List the conditions for the Hardy-Weinberg principle to apply.

Large population, random mating, no mutations, no selection, no migration.

Equation for allele frequencies?

p + q = 1, where p is the dominant allele frequency and q the recessive.

What does 2pq represent?

The frequency of heterozygotes (carriers).

What does q^2 represent?

The frequency of the homozygous recessive genotype, which is also the frequency of the recessive phenotype.

Step one of a Hardy-Weinberg calculation?

Find q^2 from the frequency of the recessive phenotype, then take the square root to get q.

Why can selection against a recessive genotype invalidate the model?

Recessive alleles are passed on less often, so allele frequencies change between generations.

The Hardy-Weinberg principleAQA A-Level Biology: Flashcards

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Define a population.

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Define a population.: A group of organisms of the same species occupying a particular space at a particular time that can potentially interbreed.
Define gene pool.: All the alleles of all the genes in a population at a given time.
What is allele frequency?: The proportion of all copies of a gene in a population that are a particular allele (a value between 0 and 1).
What does the Hardy-Weinberg principle predict?: That allele frequencies stay constant from generation to generation when its conditions apply.
List the conditions for the Hardy-Weinberg principle to apply.: Large population, random mating, no mutations, no selection, no migration.
Equation for allele frequencies?: $p + q = 1$ , where $p$ is the dominant allele frequency and $q$ the recessive.
Equation for genotype frequencies?: $p^2 + 2pq + q^2 = 1$ .
What does $2pq$ represent?: The frequency of heterozygotes (carriers).
What does $q^2$ represent?: The frequency of the homozygous recessive genotype, which is also the frequency of the recessive phenotype.
Step one of a Hardy-Weinberg calculation?: Find $q^2$ from the frequency of the recessive phenotype, then take the square root to get $q$ .
1 in 2500 people have a recessive disorder. What is $q$ ?: $q = \sqrt{1/2500} = 0.02$ .
Why can selection against a recessive genotype invalidate the model?: Recessive alleles are passed on less often, so allele frequencies change between generations.

Exam questions on The Hardy-Weinberg principle

A conservation biologist is studying rabbits living on a small island. She records the alleles present in the rabbit population for a gene controlling fur colour and wants to predict the allele frequencies in future generations.
State two conditions that must apply for the biologist's prediction, based on the Hardy-Weinberg principle, to be valid.2 marks
Cystic fibrosis is a human genetic disorder caused by a recessive allele, f. The dominant allele, F, is normal. In one population the incidence of cystic fibrosis is 1 in 2500 births. Assume that the Hardy-Weinberg principle applies to this population.
Many people with cystic fibrosis die young or do not have children. Explain why the Hardy-Weinberg principle may not predict the frequency of carriers accurately in future generations.2 marks
A population of 800 plants of one species has two alleles for flower colour. The allele for red flowers, R, is dominant to the allele for white flowers, r. In the population, 128 of the plants have white flowers. The population is assumed to be in Hardy-Weinberg equilibrium.
Calculate the frequency of the allele R and the number of heterozygous plants in the population. Show your working.3 marks

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Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).