Population genetics is the study of changes in gene frequencies in entire populations. The Hardy-Weinberg principle (H-W principle) provides a standard for an ideal population in which evolution does not occur.
Basic Knowledge
Hardy-Weinberg principle (H-W principle): In an ideal population satisfying the following five conditions, gene frequencies do not change from one generation to the next. ① A sufficiently large population ② Random mating ③ No mutation ④ No natural selection ⑤ No immigration or emigration.
H-W equations: If the frequency of allele A is p and that of allele a is q, then (p + q = 1), and genotype frequencies are: AA = p², Aa = 2pq, aa = q², and p² + 2pq + q² = 1.
Factors that change gene frequencies (driving forces of evolution): ① Mutation (produces new alleles) ② Genetic drift (chance fluctuations in allele frequencies in small populations; bottleneck effect and founder effect) ③ Natural selection (differences in fitness change allele frequencies) ④ Gene flow (movement of individuals).
In a population, the frequency of genotype aa was 0.09. Assuming Hardy-Weinberg equilibrium, determine the frequencies of alleles A and a and the frequencies of genotypes AA and Aa.
Answer: Since q² = 0.09, q = 0.3. p = 1 - 0.3 = 0.7. AA = p² = 0.49, and Aa = 2pq = 2 × 0.7 × 0.3 = 0.42. Check: 0.49 + 0.42 + 0.09 = 1.00.
- H-W principle = gene frequencies do not change in an ideal population
- H-W equations: AA=p², Aa=2pq, aa=q²
- p+q=1, p²+2pq+q²=1
- Mutation = source of new alleles
- Genetic drift = chance fluctuations in allele frequencies in small populations
- Bottleneck and founder effects = typical examples of genetic drift
- Natural selection = differences in fitness change allele frequencies
Points to Note
① The H-W principle describes a state in which evolution is not occurring. Actual populations deviate from H-W equilibrium, and this deviation is evidence of evolution. ② Genetic drift has little effect in large populations and a large effect in small populations. ③ The bottleneck effect (a temporary sharp decrease in population size) and founder effect (a small number of individuals establishing a new habitat) reduce genetic diversity.
Practice
- State the five conditions required for Hardy-Weinberg equilibrium.
- When allele frequencies are p = 0.6 and q = 0.4, determine the genotype frequencies AA, Aa, and aa under H-W equilibrium.
- What is the phenomenon in which allele frequencies fluctuate by chance in a small population?