A-LevelBiologyInherited change9700/51 · Oct/Nov 2019 · Q210 Marks

A fungal pathogen infects melon plants and causes the leaf cells to lose turgor. The melon plant then becomes permanently wilted.

(a)Suggest a tissue in which the fungal pathogen may be found and suggest a reason for your answer. tissue reason[1]
(b)Breeding experiments between a pure-breeding resistant variety of melon plant, Hemed, and a pure-breeding non-resistant variety, Dulce, were carried out to find out how resistance to the fungal pathogen is inherited. All the F1 and F2 plants were inoculated with the fungal pathogen. The numbers of resistant and non-resistant plants were counted 18 days after inoculation with the fungal pathogen. [Figure 2.1] shows the results of this large-scale breeding programme, involving several thousand melon plants. parental generation Hemed x Dulce all resistant all non-resistant F1 cross first generation (F1) x first generation (F1) all resistant all resistant second generation (F2) 13 resistant: 3 non-resistant Fig. 2.1 The results in the F2 generation gave a ratio of 13 resistant: 3 non-resistant. The results in the F2 generation were considered to be anomalous. Explain why the results for the F2 generation were considered to be anomalous and suggest a reason for this ratio.[2]
(c)(i)Outline how microarrays are used to detect mRNA in studies of gene expression in resistant and non-resistant plants.[3]
(c)(ii)Calculate the ratio of the number of genes expressed at 24 hours for the resistant variety of melon and the non-resistant variety of melon. Give your answer in whole numbers. ratio[1]
(c)(iii)Calculate the percentage change of the genes expressed between 24 hours and 48 hours in the resistant variety. percentage change =[1]
(c)(iv)State what the results in Table 2.1 indicate about resistance to the fungal pathogen.[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 (Oct/Nov 2019). The mark or mark type is shown in brackets.

[1] any one from: tissue: xylem, epidermis of root, cortex of root / parenchyma in root / endodermis, epidermis of leaf, mesophyll (of leaf); reason: transports water (for xylem), (too much) water loss (for epidermis/mesophyll)
[2] it is not a 3:1 ratio (expected) ; reason (1 of:) due to interactions between genes ; non resistant plants / certain allele combinations, have reduced chances of survival;
[3] any 3 of: 1 probes / known DNA sequences attached to plate / slide / AW ; 2 ref. to (probe) DNA is single stranded ; 3 mRNA from resistant and non-resistant plants are extracted; 4 mRNA converted to cDNA ; 5 cDNA / single strand of DNA (from samples) labelled with different colour (fluorescent) dyes (for resistant and non-resistant plants); 6 cDNA/single strand of DNA (from sample), allowed to hybridise with probe / added to plates / added to probes ; 7 fluorescence identifies presence of genes ;
[1] 3:1 ;
[1] 67 / 66.6;
[2] at 24 hours, genes expressed more rapidly / more genes expressed, in resistant variety compared to non-resistant variety / ora or at 48 hours, less genes expressed, in resistant variety compared to non-resistant variety / ora ; number of genes expressed decreases in resistant variety as time increases ;