MCAT Content / Analytic Methods / Hardy Weinberg Principle

### Hardy–Weinberg Principle

Topic: Analytic Methods

The Hardy-Weinberg principle states that a population’s allele and genotype frequencies will remain constant throughout generations; it assumes that in a given population, the population is large and is not experiencing mutation, migration, natural selection, or sexual selection.

Hardy-Weinberg equilibrium is achieved when the gene frequencies in a population do not change over time. This means the population is not evolving. There are five conditions for Hardy-Weinberg equilibrium.

1. no mutations
2. no immigration/emigration
3. no natural selection
4. no sexual selection
5. a large population

The Hardy-Weinberg principle can be used to estimate the frequency of alleles and genotypes in a population. The frequency of alleles can be estimated by calculating the frequency of the recessive genotype, then calculating the square root of that frequency to determine the frequency of the recessive allele. The frequency of alleles in a population can be represented by:

p + q = 1

where p= the frequency of the dominant allele; and q= the frequency of the recessive allele

The frequency of genotypes in a population can be represented by:

p2+2pq+q2= 1

where p2= the frequency of the homozygous dominant genotype; 2pq= the frequency of the heterozygous genotype; and q2= the frequency of the recessive genotype

Practice Questions

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Key Points

• The Hardy-Weinberg principle assumes that in a given population, the population is large and is not experiencing mutation, migration, natural selection, or sexual selection.

• The frequency of alleles can be estimated by calculating the frequency of the recessive genotype, then calculating the square root of that frequency to determine the frequency of the recessive allele.

• The frequency of alleles in a population can be represented by p + q = 1, with p equal to the frequency of the dominant allele and q equal to the frequency of the recessive allele.

• The frequency of genotypes in a population can be represented by p2+2pq+q2= 1, with p2 equal to the frequency of the homozygous dominant genotype, 2pq equal to the frequency of the heterozygous genotype, and q2 equal to the frequency of the recessive genotype.

Practice Questions

Key Terms

genotype: the combination of alleles, situated on corresponding chromosomes, that determines a specific trait of an individual, such as “Aa” or “aa”

phenotype: the appearance of an organism based on a multifactorial combination of genetic traits and environmental factors, especially used in pedigrees

allele: a variant form of a gene

mutation: a permanent alteration in the DNA sequence that makes up a gene

natural selection: the survival and reproduction of individuals due to differences in phenotype

Hardy–Weinberg equilibrium: states that allele and genotype frequencies in a population will remain constant from generation to generation in the absence of other evolutionary influences

gene frequency: the proportion of a population that carries one type of allele

recessive: able to be covered up by a dominant trait

dominant: a relationship between alleles of a gene, in which one allele masks the expression (phenotype) of another allele at the same locus

homozygous: of an organism in which both copies of a given gene have the same allele

heterozygous: of an organism which has two different alleles of a given gene

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