A-LevelBiologyInherited change9700/43 · May/June 2019 · Q213 Marks

Albinism is a condition that results from disruption of a biosynthetic pathway involving the enzyme tyrosinase. It is estimated that 1 in 17000 people in the world has albinism.

(a)A recessive mutation in the TYR gene, which codes for the enzyme tyrosinase, is one cause of albinism. Individuals with this form of albinism are homozygous recessive. Describe the phenotype of a person with albinism.[2]
(b)(i)Explain why Fig. 2.1 supports sex-linked inheritance of ocular albinism.[3]
(b)(ii)Draw a genetic diagram to show how individuals 1 and 2 cannot have a child with ocular albinism. key to symbols parental genotypes gametes offspring genotypes offspring phenotypes[4]
(b)(iii)Ocular albinism may be caused by a base deletion mutation. This mutation results in a non-functional protein. Explain how a base deletion mutation can result in a non-functional protein.[2]
(b)(iv)Ocular albinism is a non-progressive disorder and clarity of vision remains stable throughout life. A female has a family history of ocular albinism but she does not have any symptoms. A test to find out if she has the mutant allele is available. Suggest one reason for taking this test and one reason against taking this test. for against[2]

This question uses a diagram from the original paper, which isn't shown here. Open the full paper from the past papers archive to see it.

Mark scheme

Marking points from the official mark scheme (May/June 2019). The mark or mark type is shown in brackets.

[2] (a) any two from: 1 no melanin; 2 white hair; 3 (very) pale skin ; 4 pink / very pale / AW, eyes ;
[3] (b)(i) any three from: 1 more males / fewer females, have ocular albinism ; 2 males with (one copy of) recessive allele have ocular albinism / females with (one copy of) recessive allele are carriers ; 3 males (with ocular albinism) receive recessive allele from mothers; 4 fathers cannot pass on ocular albinism to their sons ; 5 (because) males pass Y chromosome to sons ; 6 males with ocular albinism pass recessive allele to daughters ; 7 detail from Fig. 2.1; e.g. all daughters (3, 5, 7) of male 1 are carriers / all sons (9 and 10) of male 1 do not have ocular albinism / males 11, 12, 13, get ocular albinism from mothers 3 and 5
[4] (b)(ii) symbols normal allele = A OA1 allele = a; parental genotypes XAY x XAXA gametes XA Y and (all) XA; offspring genotypes (phenotypes correctly linked) XAXA XAY; normal/carrier, female normal male ;
[2] (b)(iii) any two from: 1 frameshift / every triplet from site of deletion is changed ; 2 primary structure / tertiary structure / 3D shape, is changed or (protein) folded incorrectly; 3 (protein has) changed, binding / active, site; 4 premature STOP codon leads to incomplete protein;
[2] (b)(iv) any one from: for 1 decide whether to have children ; 2 earlier treatment if child born with ocular albinism ; any one from: against 3 test may be expensive ; 4 condition, is mild / not life threatening; 5 can't be treated;

On Oracle Prep you can write or photograph your own answer and have it marked by AI against this mark scheme, with feedback on what you missed.