A-LevelBiologyInherited change9700/43 · May/June 2017 · Q79 Marks

The stems of raspberry plants have spines. [Figure 7.1] shows part of a raspberry plant. [Figure 7.1] The colour of the spines is controlled by two genes, A/a and B/b. The two genes are on different pairs of chromosomes. * Allele A produces a pink anthocyanin pigment in the spines. * Allele B has no effect by itself, but increases the colour produced by allele A to give red spines. * Alleles a and b have no effect on colour. * In the absence of anthocyanin, the spines are green.

(a)State the colour of the spines of raspberry plants with the genotypes Aabb and aaBB.[2]
(b)Plants with the genotype AaBb were crossed with plants with the genotype aabb. The resulting seeds were sown and the seedlings grown until their stems developed spines. Use a genetic diagram to show the outcome of this cross, including the ratio of offspring phenotypes.[5]
(c)Suggest why the ratio you have given in your genetic diagram would be different if the genes A/a and B/b were on the same homologous pair of chromosomes.[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 (May/June 2017). The mark or mark type is shown in brackets.

[2] Aabb - pink;
aaBB - green ;
[5] 1 parents phenotypes | red | green ;
2 gametes | AB Ab aB ab | X | ab ;
3 offspring genotypes | AaBb | Aabb | aaBb | aabb ;
4 offspring phenotypes (must be linked) | red spines | pink spines | green spines | green spines ;
5 ratio | 1 | : | 1 | : | 2 ;
ecf mp 3 derived from incorrect 2 mp 4 matching incorrect 3 mp 5 matching incorrect 4
[2] max 2 of:
1 genes would be, linked / inherited together ;
2 no independent assortment ;
3 ratio 1:1 / only two classes (of phenotypes) ; A red and green or pink and green
4 rare cross-over events / recombination (gives small numbers of third phenotype) ;

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