A-LevelBiologyInherited change9700/42 · May/June 2025 · Q314 Marks

Rice, Oryza sativa, is an important grain crop. A rice grain is a seed and can have a structure known as an awn, which projects from the tip of the grain. [Figure 3.1] shows a rice grain with an awn present and a rice grain with no awn present.

(a)(i)The development of awns is controlled by two genes: gene A/a and gene B/b. The genes are present on different autosomes. The presence of either dominant allele A or dominant allele B results in rice plants producing grains with awns. A cross was carried out between a rice plant that is homozygous dominant for the two genes (double homozygous dominant) and a rice plant that is homozygous recessive for the two genes (double homozygous recessive). All F1 offspring plants produced grains with awns. Construct a genetic diagram, including a Punnett square, to show the cross that produces the F2 generation, including phenotypes. State the ratio of the offspring phenotypes produced.[5]
(a)(ii)Deduce the type of inheritance shown by the ratio of the offspring phenotypes stated in 3(a)(i).[1]
(b)It is common for wild rice plants to have grains with awns present. Suggest a selective advantage to wild rice of having awns present on their grains.[1]
(c)(i)One of the changes that occurred during the domestication of wild rice to cultivated rice was the loss of the awns from rice grains. Farmers found that long awns made storing and processing rice grains more difficult. It was also observed that rice plants that have grains with no awns have an increased grain yield. Explain the principles used by farmers to produce rice plant grains with no awns.[4]
(c)(ii)The normal allele for the gene An-1 codes for a transcription factor that has a role in awn development and in the number of grains of rice produced. When the transcription factor is present there is: • an increase in the expression of genes involved in awn development (positive regulation) • a decrease in the expression of genes involved in the number of grains produced (negative regulation). Suggest and explain how changes at the An-1 locus can cause rice plants to have grains with no awns and an increased grain yield.[3]

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 2025). The mark or mark type is shown in brackets.

[5] (a)(i) F1 genotypes: AaBb x AaBb; gametes: AB Ab aB ab (AB Ab aB ab); F2 genotypes: [Punnett square showing AB, Ab, aB, ab gametes on both axes, and resulting genotypes: AABB, AABb, AaBB, AaBb, AABb, AAbb, AaBb, Aabb, AaBB, AaBb, aaBB, aaBb, AaBb, Aabb, aaBb, aabb with corresponding phenotypes 'awn' or 'no awn'] F2 phenotypes linked to genotypes (see Punnet Square); ratio of F2 phenotypes: 15:1 awn: no awn;
[1] (a)(ii) autosomal dominant / epistasis ; I dihybrid
[1] (b) any one from: for seed dispersal ; idea of protection; anchor into the ground; obtain more, nutrients / water ;
[4] (c)(i) any four from: 1 artificial selection / selective breeding / humans act as selection pressure ; 2 select (rice) plants / seeds / grains, with no awns ; 3 idea of breeding (selected) plants / self pollination of (selected) plants ; 4 seeds germinate / plants grow / offspring grow; 5 repeat (selection and breeding) over generations ; 6 AVP; e.g. ref. to genetic technology detail of pollination
[3] (c)(ii) any three from: 1 mutation/ change in base sequence, (in An-1) or An-1 not expressed; 2 transcription factor has different, primary structure / tertiary structure / 3D shape or transcription factor not, produced / functional or transcription factor faulty ; 3 transcription factor does not bind to, the promoter / DNA ; 4 decrease in expression of gene(s) for awn development ; 5 increase in expression of gene(s) for number of grains produced ;