(a) Explain how different types of gene mutation can affect the phenotype and outline the effects of the mutant alleles that cause Huntington's disease on the phenotype of a person. (b) Explain how gibberellin acts on DELLA proteins to stimulate the production of amylase in a germinating seed.

Mark scheme

Marking points from the official mark scheme (May/June 2021). The mark or mark type is shown in brackets.

1. [9] any nine from: gene mutation 1 base substitution; 2 (often) does not have a significant effect on phenotype / silent mutation; 3 base, insertion / deletion leads to, frame shift / described ; 4 (so) has significant effect on phenotype ; 5 change in, primary structure / amino acid sequence / polypeptide made ; 6 change in, tertiary structure / 3D shape / folding; 7 loss of function in protein or enzyme / example described ; 8 (premature) stop codon ; Huntinton's disease 9 (mutant allele) is dominant ; 10 HD / dominant, allele has more repeats of base triplet CAG (than normal); 11 heterozygote will have disease ; 12 brain cells die more rapidly (than normal) / brain degeneration; 13 involuntary movements / mental deterioration or described / mood changes ; 14 onset in middle age / idea that no change in phenotype in earlier life ; 15 AVP; e.g. greater number of CAG repeats affects, earlier onset / severity of disease
2. [6] any six from: 1 DELLA proteins inhibit, transcription factor / PIF; 2 gibberellin binds to receptor ; 3 in aleurone layer; 4 ref. to enzyme involved ; 5 DELLA proteins broken down ; 6 TF / PIF, binds to promoter region (of DNA); 7 transcription of gene coding for amylase / AW ; 8 ref. to translation (leading to amylase production);