Phenotype, genotype and the environmentEdexcel A-Level Biology A: Mind map

Exam questions on Phenotype, genotype and the environment

1. A gardener takes cuttings from a single hydrangea plant with pink flowers and grows them in two soils. Cuttings grown in alkaline soil have pink flowers, but cuttings grown in acidic soil have blue flowers. All the cuttings are genetically identical to the parent plant.
   Explain why the cuttings have the same genotype but different phenotypes.2 marks
2. Mice with a particular allele of the agouti gene would normally have yellow coats and become obese. When pregnant mothers are fed a diet rich in methyl groups, many of their genetically identical offspring have brown coats and are lean. Cells from the brown mice have many more methyl groups attached to the DNA near the agouti gene than cells from the yellow mice.
   Explain how a difference in DNA methylation can lead to a difference in coat colour without a change in the base sequence of the DNA.2 marks
3. Identical twins have the same genotype. A study compared patterns of DNA methylation in twin pairs. At the age of three, the patterns in the two twins were very similar, but by the age of fifty they differed greatly. In one pair, the twin with a tumour had 72% of the CpG sites in the promoter of a tumour suppressor gene methylated, compared with 18% in the twin without a tumour.
   Explain why the methylation patterns of the twins became different as they got older, and why the differences are found in many of their cells.3 marks