Explain the relationship between genes, proteins and phenotype, with reference to two examples of genetic diseases in humans.

Mark scheme

Marking points from the official mark scheme (Oct/Nov 2025). The mark or mark type is shown in brackets.

1. [6] marks from any two groups of mark points to max 6: 1 mutant TYR gene codes for non-functioning tyrosinase ; 2 albinism / albino, recessive (disorder / allele) ; 3 lack of melanin / no melanin produced ; 4 giving, pale / fair, skin / hair or giving, transparent / pink, iris ; 5 mutant HBB gene codes for, abnormal (β) globin / haemoglobin S ; 6 sickle cell, anaemia/disease, codominant (disorder / allele); 7 less soluble / sticky / rod-forming, haemoglobin makes red blood cells sickle ; 8 sickle cells, block capillaries / cause pain (in low O2 conditions); 9 mutant F8 gene codes for non-functional factor VIII ; 10 haemophilia, recessive / sex-linked / on X chromosome; 11 fibrinogen not changed to fibrin ; 12 blood clotting problem / risk of excessive bleeding ; 13 mutant HTT gene codes for abnormal huntingtin protein ; 14 Huntington's disease dominant (disorder / allele); 15 ref. to additional / number of, CAG repeats ; 16 cognitive / movement, difficulty; other human genetic conditions (max 2 conditions with 4 possible marks each) 17 mutant named gene codes for abnormal named protein ; ; 18 type of allele that causes named condition ; ; 19 biochemical detail re. gene / protein / molecule ; ; 20 physiological effect ; ;