A-LevelBiologyInherited change9700/41 · Oct/Nov 2021 · Q79 Marks

The function of the enzyme iduronate 2-sulfatase is to break down certain complex molecules. Hunter syndrome is a rare, inherited condition in humans, caused by iduronate 2-sulfatase not functioning properly or by iduronate 2-sulfatase not being produced. This will cause the complex molecules to build up, leading to progressive damage to many organs. Explain how a gene mutation can lead to iduronate 2-sulfatase not functioning properly or not being produced.

(a)The function of the enzyme iduronate 2-sulfatase is to break down certain complex molecules. Hunter syndrome is a rare, inherited condition in humans, caused by iduronate 2-sulfatase not functioning properly or by iduronate 2-sulfatase not being produced. This will cause the complex molecules to build up, leading to progressive damage to many organs. Explain how a gene mutation can lead to iduronate 2-sulfatase not functioning properly or not being produced.[4]
(b)Hunter syndrome is caused by a recessive allele located on the X chromosome. Using appropriate symbols, draw a genetic diagram to show the probability of a normal male and a heterozygous normal female having a child with Hunter syndrome. key parental phenotype parental genotype gametes offspring genotypes offspring phenotypes probability[5]

Mark scheme

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

[1] base / nucleotide, insertion / deletion / substitution;
[1] frameshift / changed reading frame;
[1] ref. to stop codon ;
[1] changes / different, primary structure / amino acid sequence;
[1] changes / different, folding / 3D shape / 3º structure, of enzyme / protein;
[1] active site does not, bind / fit, substrate / complex molecules;
[1] key e.g. H = normal (allele) h = Hunter (syndrome allele) ;
[1] parental phenotype normal male normal female
[1] parental genotype XHY XHXh;
[1] gametes (XH Y XH Xh)
[1] offspring genotypes XH XH XH Xh XH Y Xh Y;
[1] offspring phenotypes normal female normal / carrier female normal male Hunter S male
[1] probability 0.25 / 25% / 1 in 4;

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