A-LevelBiologySelection and evolution9700/41 · Oct/Nov 2022 · Q613 Marks

Selective breeding has been used to improve the characteristics of crop plants, including maize and rice.

(a)Outline how vigorous, uniform varieties of maize were developed by selective breeding.[4]
(b)IR8 is a variety of rice plant that was developed in the 1960s. IR8 was developed by breeding together two different varieties of rice: • PETA, which produces a high yield • DGWG, which is a dwarf variety. Fig. 6.1 is a diagram showing the three varieties of rice. [Figure 6.1]
(b)(i)Suggest why IR8 is an improvement on the PETA variety of rice.[1]
(b)(ii)In 2009, scientists produced a new variety of rice, IR64-Sub1, by breeding together: • FR13A, a variety which has a low yield but has an allele for flood tolerance • IR64, a variety which produces a high yield. The scientists sequenced the DNA of these three rice varieties. Suggest the benefit of sequencing the DNA of IR64-Sub1.[1]
(b)(iii)After a few generations of breeding, the scientists crossed IR64-Sub1 with IR64. Explain why the scientists crossed IR64-Sub1 with IR64.[1]
(c)Thale cress, Arabidopsis thaliana, is another plant that has dwarf varieties. Scientists treated three varieties of A. thaliana with gibberellin: • the wild type (normal, non-dwarf variety) • dwarf variety A • dwarf variety B. Fig. 6.2 shows the responses of the three varieties to treatment with gibberellin. [Figure 6.2]
(c)(i)With reference to Fig. 6.2, suggest explanations for the different responses to gibberellin shown by dwarf variety A and dwarf variety B.[4]
(c)(ii)BZR1 is a transcription factor that helps to regulate growth and development in A. thaliana. Outline the features of a transcription factor such as BZR1.[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 2022). The mark or mark type is shown in brackets.

[4] (a) any four from: 1 breed / cross, plants with desired (named) characteristics ; 2 breed, better / best, offspring, repeatedly / for many generations ; 3 (correct ref. to) inbreeding / line breeding; 4 (correct ref. to) hybridisation / outbreeding / outcrossing; 5 (gives) hybrid vigour / heterosis / outbreeding enhancement (in offspring); 6 F1 have, same genotype / uniform phenotype ; 7 AVP;
[1] (b)(i) any one from: 1 (IR8) less likely to, fall over / lodge / collapse; 2 (IR8) higher proportion / more, of plant is, edible / grain;
[1] (b)(ii) check for, genes / alleles / DNA, for high yield and flood tolerance;
[1] (b)(iii) any one from: 1 to, maintain high yield / increase yield / give higher yield ; 2 to avoid inbreeding depression or to maintain, hybrid vigour / heterosis ;
[4] (c)(i) any four from: variety A: 1 gibberellin binds to, receptor / GID1 ; 2 DELLA protein breaks down ; 3 PIF / transcription factor (released) to, bind to DNA / transcribe genes ; 4 stem / internode / cell, elongation / growth, occurs ; 5 A, is lele / has no Le allele ; 6 A does not produce its own (active) gibberellin ; variety B: 7 does not respond to / no effect from, gibberellin; 8 lacks / has non-functional, gibberellin receptor ;
[2] (c)(ii) any two from: 1 (BZR1) binds to promoter ; 2 to switch gene on / so gene is expressed / allow transcription; 3 allows binding of RNA polymerase (to DNA / promoter);