Sweet peas are garden plants that vary in height. • Tall sweet peas grow to 200 cm in height. • Dwarf sweet peas grow to 30 cm in height. • Tall sweet peas contain a dominant Le allele. • Dwarf sweet peas are homozygous for the recessive le allele. Explain how the lele genotype results in the dwarf phenotype in sweet peas, with reference to the effect of lele on: • enzyme synthesis • hormone production • the expression of genes affecting plant growth.

Mark scheme

Marking points from the official mark scheme (May/June 2023). The mark or mark type is shown in brackets.

1. 1 le allele codes for a, non / less, functional enzyme ;
2. 2 3 beta-hydroxylase ;
3. 3 alanine replaced with threonine at active site ;
4. 4 inactive gibberellin / GA20, not converted to, active gibberellin / GA1 ;
5. 5 no / less, gibberellin to bind to, (intracellular) receptor / GID1 ;
6. 6 no / less, gibberellin-receptor-DELLA complexes formed ;
7. 7 DELLA not broken down ;
8. 8 DELLA (stays) bound to, transcription factors / PIF ;
9. 9 transcription factor / PIF / RNA polymerase, cannot, bind to promoter ;
10. 10 growth genes / XET gene, (stay) switched off / not transcribed / not expressed ;
11. 11 stem does not elongate / internode length is small OR no / less, cell division in stem ;