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Hardy-Weinberg Equilibrium Reference Cheat Sheet
A printable reference covering Hardy-Weinberg equations, allele frequencies, genotype frequencies, assumptions, and evolutionary change for grades 10-12.
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Hardy-Weinberg equilibrium describes what happens to allele and genotype frequencies in a population when evolution is not occurring. This cheat sheet helps students organize the equations, assumptions, and reasoning used in population genetics problems. It is useful for connecting Punnett square ideas to real populations and for deciding whether a population is evolving. The core formulas are p + q = 1 and p^2 + 2pq + q^2 = 1, where p and q are allele frequencies. The terms p^2, 2pq, and q^2 represent genotype frequencies for homozygous dominant, heterozygous, and homozygous recessive individuals. A population is in Hardy-Weinberg equilibrium only if there is no mutation, random mating, no natural selection, very large population size, and no gene flow.
Key Facts
- For a gene with two alleles, the allele frequency equation is p + q = 1.
- The genotype frequency equation is p^2 + 2pq + q^2 = 1.
- In Hardy-Weinberg notation, p is the frequency of one allele and q is the frequency of the other allele.
- The genotype frequency p^2 represents homozygous dominant individuals, 2pq represents heterozygous individuals, and q^2 represents homozygous recessive individuals.
- If the recessive phenotype is visible, its frequency is q^2, so q = square root of q^2.
- Allele frequencies and genotype frequencies are proportions, so they must be between 0 and 1.
- Hardy-Weinberg equilibrium requires no mutation, random mating, no natural selection, a very large population, and no migration.
- If observed genotype frequencies are very different from expected Hardy-Weinberg frequencies, the population may be evolving.
Vocabulary
- Allele frequency
- The proportion of a specific allele among all copies of that gene in a population.
- Genotype frequency
- The proportion of individuals in a population that have a specific genotype.
- Hardy-Weinberg equilibrium
- A condition in which allele and genotype frequencies stay constant across generations because no evolutionary forces are acting.
- Homozygous
- Having two identical alleles for a gene, such as AA or aa.
- Heterozygous
- Having two different alleles for a gene, such as Aa.
- Gene flow
- The movement of alleles into or out of a population through migration and reproduction.
Common Mistakes to Avoid
- Using q instead of q^2 for the recessive phenotype is wrong because the recessive phenotype usually represents homozygous recessive individuals, not the recessive allele frequency.
- Forgetting to take the square root of q^2 is wrong because q^2 is a genotype frequency, while q is an allele frequency.
- Treating percentages as whole numbers is wrong because Hardy-Weinberg equations use proportions, so 36% must be written as 0.36.
- Assuming every population is in equilibrium is wrong because Hardy-Weinberg equilibrium depends on strict conditions that are often not met in nature.
- Mixing up 2pq and p^2 is wrong because 2pq represents heterozygotes, while p^2 represents homozygous dominant individuals.
Practice Questions
- 1 In a population, 16% of individuals show the recessive phenotype. Find q, p, and the expected heterozygote frequency 2pq.
- 2 A population has p = 0.7 and q = 0.3. Calculate the expected frequencies of AA, Aa, and aa.
- 3 In a sample of 500 individuals, the expected aa frequency is 0.09. How many individuals are expected to have the aa genotype?
- 4 A small island population loses many individuals after a storm. Explain why this population may no longer meet Hardy-Weinberg equilibrium conditions.