Phenotype, genotype and the environmentEdexcel A-Level Biology A: Subtopic test

1. 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.

   (a)

   Which statement best explains the difference in flower colour between the cuttings?

   [1 mark]

   • AThe cuttings have different genotypes
   • BThe phenotype is the result of an interaction between the genotype and the environment
   • CThe acidic soil changes the base sequence of the DNA of the gene
   • DMutations happen only in cuttings grown in alkaline soil

   (b)

   A cutting from a pink-flowered plant is moved from alkaline soil to acidic soil. What is the most likely result, and why?

   [1 mark]

   • APink flowers, because the genotype is fixed so the phenotype cannot change
   • BBlue flowers, because the acidic soil mutates the gene
   • CPink flowers, because the colour is always inherited permanently
   • DBlue flowers, because the same genotype can produce a different phenotype in a different environment

   (c)

   Explain why the cuttings have the same genotype but different phenotypes.

   [2 marks]

   Total for question 1: 4 marks

2. 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.

   (a)

   What is meant by DNA methylation?

   [1 mark]

   • AMethyl groups are added to the DNA, usually reducing the transcription of the gene
   • BAcetyl groups are added to the histones, so DNA winds tightly
   • CThe base sequence of the gene is changed by the substitution of methyl groups for bases
   • DMethyl groups are added to mRNA so it cannot be translated

   (b)

   Which statement about histone modification is correct?

   [1 mark]

   • AAdding acetyl groups to histones makes DNA wind more tightly, so transcription decreases
   • BHistone modification changes the base sequence of the gene
   • CRemoving acetyl groups from histones makes chromatin more condensed, so transcription decreases
   • DHistone modification affects only mRNA after transcription

   (c)

   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]

   Total for question 2: 4 marks

3. 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.

   (a)

   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]

   (b)

   Calculate how many times greater the methylation of the promoter was in the twin with the tumour than in the twin without. Suggest how this difference could have contributed to the development of the tumour.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A researcher measures the adult height of 5000 people and finds that the heights show a bell-shaped distribution with no distinct categories. She also studies pairs of identical twins who were separated at birth and brought up in different homes: on average their heights differ by 2.1 cm, whereas the heights of unrelated adults of the same sex differ by about 9 cm.

   (a)

   Explain how polygenic inheritance and the environment together produce continuous variation in a characteristic such as height.

   [6 marks]

   (b)

   Evaluate the claim that a person's height is determined entirely by their genotype. Include a justified conclusion.

   [6 marks]

   Total for question 4: 12 marks

End of questions