Steelhead trout, Oncorhynchus mykiss, are fish that live in streams in North America. To increase the number of steelhead trout, captive breeding has occurred since 1992. Fish eggs and sperm are mixed and the young fish grow in large tanks of aerated water for the first year of their lives. Most are then released into the wild, however a few male and female fish are kept to become the parents of the next generation of captive-bred fish. Each tank may hold up to 50000 fish. The young captive fish are fed processed food. Some young fish are unable to survive these conditions and a proportion die. Death is usually the result of poor wound-healing after accidents due to overcrowding and due to the spread of diseases.

Mark scheme

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

1. [1] continuous; A normal distribution
2. [1] evolution / natural selection / artificial selection / directional selection / adaptation / selective breeding;
3. [3] any three from: captive fish:
4. 1 controlled conditions so phenotype less varied ;
5. 2 more oxygen so larger ;
6. 3 less space / overcrowding, so smaller ;
7. 4 less space / overcrowding, so (signs of) disease / wounds ;
8. 5 food qualified linked to, size / muscle / fat ;
9. 6 AVP; e.g. named tank condition linked to skin colour / no predators linked to increase in size / temperature linked to size
10. [5] any five from:
11. 1 obtain mRNA from wild and captive bred fish / AW ;
12. 2 reverse transcription of mRNA to produce cDNA ;
13. 3 add fluorescent label to (c)DNA; I colour A dyes/markers / tags
14. 4 (microarray has) ssDNA probes ;
15. 5 each from a, different / known, gene;
16. 6 (c)DNA hybridises / AW, to, probes / ssDNA (on microarray);
17. 7 fluorescence shows the expressed genes ;
18. 8 intensity of fluorescence (shows level of gene expression) ;
19. 9 compare, fluorescence / gene expression (between fish groups);
20. 10 AVP; e.g. ref. to washing off excess (c)DNA (only), after hybridisation UV light / laser scanning (to record fluorescence pattern)
21. [4] any three from:
22. 1 ref. to transcription factors ;
23. 2 TF / gene product / protein, bind to DNA ;
24. 3 (TF binds to) promoter ;
25. 4 switches genes, on / off ; AW I genes, expressed / not expressed
26. 5 ref. to binding of RNA polymerase ; R if operator mentioned
27. 6 mRNA is made / transcription occurs ; ora
28. 7 AVP; e.g.(so that genes are expressed) at the correct time / in the correct context / in the correct cell type / in the correct order;
29. [1] any one from:
30. 1 differences caused by selection in captive-bred population ;
31. 2 (not environmental because) all, offspring / fish, were kept in same environment ;
32. [1] idea that captive fish have better, wound healing / immune response ;