2019HSCShort answer

Q33 (b) (ii)

4 marks

Question

Alzheimer’s disease causes destruction of brain tissue, dementia and eventually death. The diagram shows the effect of Alzheimer’s disease on the brain.
The gene with the greatest known effect on the risk of developing late-onset Alzheimer’s disease is called APOE. It is found on chromosome 19.
The APOE gene has multiple alleles, including e2, e3 and e4.
The table shows the risk of developing Alzheimer’s disease for various APOE genotypes compared to average risk in the population.

Analyse the data to assess the risk of developing Alzheimer’s disease associated with the e2, e3 and e4 alleles.

Uncropped source page 28 for question 33.b.ii; requires human review and cropping — Reviewed visual · source page 28

Uncropped source page 29 for question 33.b.ii; requires human review and cropping — Reviewed visual · source page 29

Try the question before revealing the answer.

Solution

Criteria	Marks
Analyses the data to provide relevant conclusions about the risk associated with the alleles and combinations of alleles	4
Analyses the data to provide suitable conclusions about the risk associated with the alleles	3
Outlines risks associated with alleles	2
Provides some relevant information	1

Sample answer:

The presence of e2 reduces the risk of AD. When it is present in the homozygous genotype or heterozygous with e3 there is a 40% reduction in risk of AD. The e2 allele appears to mask the effect of the e3 allele (e2 is dominant over e3). When e2 is combined with e4 AD is 2.6 times more likely, suggesting that e2 cannot fully mask e4. If two e4 alleles are present the risk of AD is greatly increased and is 14.9 times more likely. This suggests that e2 reduces the risk of AD and that e4 significantly increases the risk of AD.

When the individual is homozygous for the e3 allele there is an average risk of AD. However when e3 and e4 are present together the presence of e3 appears to reduce the increased risk that is the result of the e4 allele (e3/e4 makes AD 3.2 times more likely).

Supplied marking guidance · human reviewed

Compare this with your working, not only your final sentence.

Syllabus alignment

Module 5 · Heredity

DNA and Polypeptide SynthesisAssess how genes and the environment affect phenotypic expression.
Genetic VariationAnalyse frequencies of population characteristics and single nucleotide polymorphisms.

Spot a problem? Send feedback