Genome-wide association studies find links between single nucleotide polymorphisms (SNPs) and phenotypic features such as human diseases. SNPs are points on the DNA that vary in the population because of DNA base substitutions. A genome-wide association study investigates the effect of genetic variation on a disease. A large number of people with the disease and a large number of healthy control individuals provide DNA. Microarray chips are used to identify the genotype of each individual at many SNPs. The Wellcome Trust Case Control Consortium (WTCCC) study was an important genome- wide association study. • The study used a microarray chip that identified each person's genotype at 500 000 different SNPs. • The study looked for links between SNPs and 7 different diseases. • For each disease, 2000 people with the disease were tested. • Their results were compared with the results of 3000 healthy control individuals.

Mark scheme

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

1. [1] (a)(i) obtain single-stranded DNA ;
2. [1] (a)(i) label DNA with fluorescent, dye / tag / label;
3. [1] (a)(i) probes on, chip / microarray;
4. [1] (a)(i) each probe is unique to a particular, SNP / gene / allele;
5. [1] (a)(i) DNA (added), binds / hybridises, to probes ;
6. [1] (a)(i) wash off excess DNA (after hybridisation);
7. [1] (a)(i) (view under) UV light / laser scanner / high resolution digital camera;
8. [1] (a)(i) fluorescence indicates presence of, SNP / gene; R gene expressed
9. [1] (a)(ii) large database ;
10. [1] (a)(ii) use database to find, SNPs / probes ;
11. [1] (a)(ii) fast/accurate / efficient ;
12. [1] (a)(ii) ref. to (computer) software / algorithms ;
13. [1] (a)(ii) ref. to statistics / statistical analysis ;
14. [1] (a)(ii) idea of, 17 000 × 500 000 / 8.5 × 10⁹, data points;
15. [1] (b)(i) 1 and 6;
16. [1] (b)(ii) all diseases, are autosomal / not associated with X chromosome / not sex-linked ;
17. [1] (b)(ii) all diseases are associated with SNP on chromosome 6;
18. [1] (b)(ii) type 1 and type 2 diabetes SNPs are associated with chromosomes 12 and 16 (but not rheumatoid arthritis);
19. [1] (b)(ii) type 2 diabetes SNP associated with chromosome 10 (but not type 1 diabetes and rheumatoid arthritis);
20. [1] (b)(ii) arthritis and type 1 diabetes have higher level of association (with each other) than with type 2 diabetes ;
21. [1] (b)(ii) rheumatoid arthritis has fewest number of SNPs or type 1 and type 2 diabetes has more SNPs (than rheumatoid arthritis);
22. [1] (c) DNA analysis not available for everyone ;
23. [1] (c) lifestyle change; A e.g. lose weight / stop smoking / take exercise
24. [1] (c) early treatment;
25. [1] (c) allows people to plan; e.g. organise care / appoint power of attorney / take out medical insurance / make will / gift property to children / retire early
26. [1] (c) decide whether to have children / ref. (therapeutic) termination;
27. [1] (c) results may cause, anxiety / stress / panic / depression (if positive) or reduce worry (if negative);
28. [1] (c) results may affect ability to get, insurance / credit / jobs ;
29. [1] (c) idea of predictions may not be accurate ;