OxfordAQA International A Level Biology 3.3.7 Allele Frequencies in Populations Exam Questions

About OxfordAQA International A Level Biology 3.3.7 Allele Frequencies in Populations

Allele frequencies in populations in Unit 3 applies genetics to whole populations. You need to define a population, gene pool and allele frequency, and explain how selection has been used to produce high-yielding breeds of domesticated animals and cultivated plants. You also need to explain genetic bottlenecks and genetic drift, especially in small populations. The main calculation is the Hardy-Weinberg principle: you must know the conditions under which it applies and use the equations to calculate allele, genotype and phenotype frequencies. It links back to inheritance and genetic diversity and forward to evolution and speciation. Hardy-Weinberg calculations appear frequently.

What you need to know

• Gene pool A gene pool is all the alleles of all the genes in a population at one time. Allele frequency is the proportion of a particular allele in the gene pool.
• Selective breeding Humans select individuals with desired characteristics, such as high yield, and breed from them over many generations. This changes allele frequencies and reduces genetic diversity.
• Genetic bottlenecks A large reduction in population size, caused by an environmental event or human activity, leaves few survivors. The surviving population has reduced genetic diversity, and its allele frequencies may differ from the original population.
• Genetic drift Genetic drift is a change in allele frequency due to chance. It has a bigger effect in small populations and when there are few copies of an allele.
• Hardy-Weinberg p + q = 1 and p^2 + 2pq + q^2 = 1, where p is the frequency of the dominant allele and q the recessive allele. The principle assumes a large population, random mating, no mutation, no migration and no selection.

Common mistakes in 3.3.7 Allele Frequencies in Populations

• Using the frequency of the dominant phenotype as p^2. The dominant phenotype is p^2 + 2pq; always start from q^2, the homozygous recessive frequency.
• Confusing allele and genotype frequencies. p and q are allele frequencies; p^2, 2pq and q^2 are genotype frequencies.
• Saying genetic drift is caused by selection. Genetic drift is change due to chance, not because some alleles give an advantage.
• Forgetting the conditions of the Hardy-Weinberg principle. Small populations, migration or selection mean the equation will not predict accurately.

Try three 3.3.7 Allele Frequencies in Populations exam-style questions