A-LevelBiologySelection and evolution9700/42 · May/June 2023 · Q510 Marks

(a) Warfarin is a poison used to kill rats. Some rats are resistant to warfarin and can survive the effects of the poison. Warfarin resistance is due to a mutant dominant allele at a single gene locus. Researchers investigated the population of brown rats on a large island where there are no predators of rats. On this island, warfarin is used to try to control the rat population. The researchers found that 40% of the population of rats was resistant to warfarin.

(a)(i)Use Hardy-Weinberg equation 1 and equation 2 to calculate the percentage of the population of rats on the island that is heterozygous for warfarin resistance. equation 1 p + q = 1 equation 2 p² + 2pq + q2 = 1 Show your working. answer % [3][3]
(a)(ii)In natural populations, it is usual to find that only some of the conditions of the Hardy-Weinberg principle are met. Suggest and explain which of the conditions of the Hardy-Weinberg principle are most likely to be met for the island population of brown rats.[4]
(b)Dominant advantageous alleles and recessive advantageous alleles both naturally occur in populations. Explain why, when a new dominant advantageous allele occurs, its frequency increases more quickly in the population than when a new recessive advantageous allele occurs.[3]

Mark scheme

Marking points from the official mark scheme (May/June 2023). The mark or mark type is shown in brackets.

[3] q2 = 0.6 or q = √ 0.6 or q = 0.775 or p = 0.225; 2pq = 2 × 0.225 × 0.775 or 0.35 ; allow ecf from mp1 35%; allow ecf from mp2
[4] any four from: 1 no migration or no flow of, alleles / genes, into / out of, the population; 2 population is isolated / described; e.g. separated by geographical barrier I ref to island alone 3 random mating is occurring ; 4 rats move around island / AW ; 5 large population ; 6 rats have high reproductive rate / no predators; 7 rats are diploid; 8 have two different alleles (for the gene) / heterozygous ;
[3] any three from: 1 new allele initially found in heterozygotes; dominant advantageous allele 2 (new) dominant allele is, expressed in heterozygote / always expressed ; 3 selection can act on individuals with dominant allele straight away / AW ; recessive advantageous allele 4 (new) recessive allele is not expressed, in heterozygote / due to dominant allele or recessive allele only expressed in homozygous recessive (genotype) ; 5 selection can only act on individuals who are homozygous recessive ; 6 it takes time for homozygous recessive (genotype) to occur / AW ;

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