A-LevelBiologySelection and evolution9700/42 · Feb/Mar 2020 · Q411 Marks

The rainbow trout, Oncorhynchus mykiss, is a fish that is bred in commercial fish farms. Rainbow trout that have a blue-silver colour are sold at a higher price than rainbow trout that have a brown colour. The number of fish with each of the different colours was recorded in a breeding population in one fish farm. • population total = 2936 • number of blue-silver fish = 1437 • number of brown fish = 1499

(a)The colour of rainbow trout is controlled by a single autosomal gene with two alleles, one dominant and one recessive. The blue-silver colour occurs when a fish is homozygous for the recessive allele. The formulae of the Hardy-Weinberg principle state that: p + q = 1 p² + 2pq + q² = 1 Use these formulae to calculate the expected number of brown fish that are homozygous, if the Hardy-Weinberg principle applies to this population. Show your working. answer[3]
(b)Some scientists suggested that the Hardy-Weinberg principle did not apply in this situation because the rainbow trout were being kept in a commercial fish farm. State two conditions that must be met by this commercial fish farm population for the Hardy-Weinberg principle to apply.[2]
(c)Describe how selective breeding can be used to increase the proportion of rainbow trout with a blue-silver colour in commercial fish farms.[4]
(d)The Atlantic salmon, Salmo salar, can be genetically modified (GM). GM Atlantic salmon are bred in commercial fish farms for food production. In 2017, GM Atlantic salmon bred in Canada became the first GM animal to enter the human food chain. Explain how the genetic modification of the Atlantic salmon can be used to increase food production.[2]

Mark scheme

Marking points from the official mark scheme (Feb/Mar 2020). The mark or mark type is shown in brackets.

[3] 4(a) (q²) = 1437/2936 = 0.489 q = square root of 0.489 = 0.700 or 0.699; p = 1 -0.700 = 0.300 (or p = 1 – 0.699 = 0.301) (p²) = 0.3002 = 0.090 or (p²) = 0.3012 = 0.091); (0.090 × 2936 or 0.091 × 2936) = 264 / 265 / 266/267; one mark for calculation of q or q², one mark for calculation of p or p² and one mark for answer A ecf
[2] 4(b) any two from: 1 population is large; 2 no migration (into or out of the population); 3 random mating occurs ; 4 undergoes sexual reproduction / ora; 5 organism is diploid ; 6 no selection; 7 no mutation;
[4] 4(c) any four from: 1 artificial selection / humans apply selection pressure ; 2 remove dark brown, O. mykiss / rainbow trout, from breeding population / ora; 3 breed, O. mykiss / rainbow trout, with shiny blue-silver appearance ; 4 over several generations ; 5 using offspring with shiny blue-silver appearance; 6 frequency of shiny blue-silver allele increases ; 7 only shiny blue-silver allele passed on to offspring;
[2] 4(d) any two from: 1 gene / allele, for, faster growth rate / growth hormone / larger sized salmon, (inserted); 2 ref. to promoter from another species (also inserted); 3 growth occurs, all year round / not just in spring and summer ; 4 GM salmon, grow faster / grow larger / reproduce more ;

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