Question 23M.2.SL.TZ2.2
| Date | May 2023 | Marks available | [Maximum mark: 5] | Reference code | 23M.2.SL.TZ2.2 |
| Level | SL | Paper | 2 | Time zone | TZ2 |
| Command term | Explain, State, Suggest | Question number | 2 | Adapted from | N/A |
The dolichos bean (Lablab purpureus) is cultivated as a food crop in tropical countries. Leaf cells in L. purpureus have 24 chromosomes.
State how many chromosomes there would be in male or female gametes of L. purpureus.
[1]
12;
No alternative.
Only just over half of candidates realised that in (a) gametes contain half the number of chromosomes of a normal cell.
L. purpureus can have purple or white flowers. Two pure-breeding varieties were crossed: HA 4 with white flowers and GL 424 with purple flowers. All of the F1 plants had purple flowers. The F1 plants were self-pollinated to produce an F2 generation. There were 97 plants with purple flowers and 38 plants with white flowers in the F2 generation.
Using a Punnett grid, explain the results of this cross.
[3]
- male and female gamete genotypes/alleles shown as P and p (or others following convention with a suitable key) in a punnet grid;
- F2 genotypes shown as PP, Pp, pP and pp;
- F2 phenotypes indicated for each genotype on the Punnett grid / 3 purple to 1 white ratio indicated;
Accept other upper-case and lower-case letters for the alleles.
No punnet grid = 2 MAX.
Allow ECF if alleles incorrect.
The approximate 3:1 ratio was not identified by many candidates. Many weaker candidates tried to work in HA 4 and GL 424 instead of using the conventional capital and lower-case letters for the alleles.
Suggest a reason for the F2 generation not corresponding exactly to the expected ratio of purple-flowered and white-flowered plants.
[1]
chance / OWWTE;
(any of the usual mechanisms causing recombination of alleles) NOT mutation.
Any wording that implies the bigger the sample size, the closer to 3:1.
The fact that fertilisation is random / subject to chance was not appreciated in (c), with the most common incorrect answer being mutation.