| Date | May 2019 | Marks available | 1 | Reference code | 19M.1.HL.TZ2.20 |
| Level | Higher level | Paper | Paper 1 | Time zone | Time zone 2 |
| Command term | Deduce | Question number | 20 | Adapted from | N/A |
Question
William Bateson and Reginald Punnett used the sweet pea (Lathyrus odoratus) in genetics studies in the early 20th century. Pure-breeding plants that produced purple flowers and long pollen grains were crossed with pure-breeding plants that produced red flowers and round pollen grains. The resulting offspring all produced purple flowers and long pollen grains. Two of the F1 generation plants were crossed. The table shows the ratio of phenotypes in the F2 generation.
What is an explanation for these experimental results?
A. Purple flowers and long pollen grains are dominant and the alleles have assorted independently.
B. The genes for flower colour and pollen shape are linked and all plants producing long pollen grains are recombinants.
C. The genes for flower colour and pollen shape are linked and all plants producing red flowers are recombinants.
D. Plants producing purple flowers and round pollen grains arose through crossing over.
Markscheme
D
Examiners report
This was a wordy question that turned out to be quite time consuming. Many candidates failed to realize that if the alleles had assorted independently the ratios would have been 9:3:3:1. The only way the results shown could have been obtained was through genetic linkage between purple and long and red and round. Purple and round are formed by recombination of alleles through crossing over.
