Question 1
Paper 2Medium16 marksPhenylketonuria (PKU) is caused by the recessive mutated allele p. The allele P is dominant, and only individuals with genotype pp have PKU. Two unaffected parents both have genotype Pp.
Their daughter also has genotype Pp. As an adult, she develops a skin cancer. A mutation, T, in a gene involved in regulating cell division is detected in her tumour cells, but not in her non-tumour skin cells, blood cells or gametes.
Non-tumour skin cells and tumour cells from the daughter were cultured under the same conditions. The table shows the mean percentage of cells that underwent division in 48 hours, from three replicate cultures.
| Cell sample | Mean percentage of cells undergoing division in 48 hours |
|---|---|
| Non-tumour skin cells | 8% |
| Tumour cells | 35% |
Determine the expected genotype ratio among the parents’ offspring and the probability that an offspring will have PKU.
Explain why the daughter with genotype Pp does not have PKU, and how she could pass allele p to a child who has PKU.
Describe the difference in cell division between the two cell samples in the table.
Using the information about mutation T, explain how it could contribute to cancer and whether it could be inherited by the daughter’s offspring.
Give one limitation of the cell-culture results in deciding whether mutation T caused the higher rate of cell division.