Question 1
Paper 2Medium16 marksIn a population of ground beetles, shell pattern is determined by one gene with two alleles, P and p. P is dominant: beetles with PP or Pp have plain shells, while beetles with pp have speckled shells. A base substitution in a germ cell changed a P allele to p, and the p allele was inherited by descendants.
Two heterozygous beetles mate. In a separate investigation, equal numbers of plain and speckled beetles were placed on dark or pale soil. The number alive after two weeks was recorded.
| Soil colour | Shell pattern | Number at start | Number alive after two weeks |
|---|---|---|---|
| Dark | Plain | 40 | 24 |
| Dark | Speckled | 40 | 32 |
| Pale | Plain | 40 | 32 |
| Pale | Speckled | 40 | 24 |
Assume shell pattern is inherited as described and does not change with soil colour. Surviving beetles are equally likely to reproduce.
Explain how the mutation described could provide heritable variation in the beetle population.
Use a genetic cross to determine the expected genotype and phenotype ratios among the offspring of the two heterozygous beetles. State how sexual reproduction generates variation in this cross.
Calculate the percentage survival of each shell pattern on each soil colour. Use the results to explain how natural selection may act on variation in this population, distinguishing the role of mutation from the role of natural selection.