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Hardy-Weinberg: what alleles do in a population when nothing acts on them questions
Gene pools and allele frequencies, the Hardy-Weinberg principle and its five conditions, the equations p plus q equals one and p squared plus two pq plus q squared equals one, calculating carrier frequency from the frequency of a recessive phenotype, and interpreting a departure from equilibrium.
6 original questions · 22 marks · the hardy-weinberg: what alleles do in a population when nothing acts on them notes · Inheritance and population genetics
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In a population of 5000 people, 450 show a condition caused by a recessive allele. Assuming the population is in Hardy-Weinberg equilibrium, calculate the frequency of each allele and the number of people expected to be heterozygous.
Mark scheme
- M1 the affected people are the homozygous recessives, so q2 = 450 ÷ 5000 = 0.09
- M1 take the square root to get the allele frequency: q = 0.3, and since p + q = 1, p = 0.7
- A1 the heterozygotes are 2pq = 2 × 0.7 × 0.3 = 0.42
- A1 0.42 × 5000 = 2100 people, and a check that 2450 homozygous dominant plus 2100 plus 450 comes back to 5000
A sample of 200 snails contains 98 of genotype CC, 84 of genotype Cc and 18 of genotype cc. Calculate the frequency of each allele in the sample, and calculate the number of heterozygotes expected under Hardy-Weinberg equilibrium in order to say whether the sample is at equilibrium.
Mark scheme
- M1 each snail carries two alleles, so the sample holds 2 × 200 = 400 alleles, and the count of C is 2 × 98 + 84 = 280
- A1 p = 280 ÷ 400 = 0.7 for C, and q = 120 ÷ 400 = 0.3 for c, which add to 1 as they must
- M1 the expected number of heterozygotes is 2pq multiplied by the number of individuals: 2 × 0.7 × 0.3 × 200
- A1 84, which is exactly the number observed, so the sample is in Hardy-Weinberg equilibrium
Explain why a dominant allele does not become more common in a population simply because it is dominant.
Mark scheme
- B1 dominance describes which allele is expressed in a heterozygous individual, and says nothing about how often the allele occurs in the gene pool
- B1 the two alleles of a heterozygote are equally likely to enter a gamete, because they separate at meiosis independently of which one is expressed
- B1 the Hardy-Weinberg principle predicts that with a large population, random mating, no selection, no mutation and no migration, allele frequencies stay constant from one generation to the next whatever the dominance relationships
- A1 the evidence agrees: polydactyly is caused by a dominant allele and is rare, while the recessive allele for blood group O is the commonest of the three
A biologist finds that the observed genotype frequencies in a population of beetles differ significantly from those predicted by its allele frequencies. Explain what this tells the biologist, and explain how the pattern of the difference helps to identify the cause.
In parts of sub-Saharan Africa the sickle-cell allele occurs at a far higher frequency than selection against affected homozygotes should allow. Suggest an explanation for this, and suggest why the same allele is rare among populations in northern Europe.
State what is meant by the gene pool of a population, and state what is meant by the frequency of an allele.
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