(a) Describe the types of gene mutation that can occur and explain how some mutations can lead to a shortened polypeptide. (b) Outline the effects of the mutant sickle cell allele on the phenotype of a person with sickle cell anaemia.

Mark scheme

Marking points from the official mark scheme (Oct/Nov 2017). The mark or mark type is shown in brackets.

1. [6] 10(a) six from: 1 base / nucleotide, substitution ; 2 missense / silent, mutation ; 3 base / nucleotide, insertion / addition ; 4 base / nucleotide, deletion ; 5 may cause frameshift ; 6 alters triplets of following, base / nucleotide, sequence ; 7 (premature) stop codon gives shortened polypeptide ; 8 does not code for amino acid ; 9 nonsense mutation ;
2. [9] 10(b) nine from: 1 homozygous for, mutant allele / HbS ; 2 altered β polypeptide in haemoglobin ; 3 haemoglobin / β-globin, less soluble ; 4 in low(er) oxygen (concentration) ; 5 (Hb) forms long fibres ; 6 red blood cells, sickle / form crescent shape ; 7 (RBCs) carry less oxygen ; 8 (RBCs) get stuck in capillaries ; 9 blocks blood flow ; 10 causes pain ; 11 sickle cell crisis ; 12 RBCs break down faster / lack of RBCs ; 13 protection against, malaria / Plasmodium infection ;