Question 22N.2.HL.TZ0.2
| Date | November 2022 | Marks available | [Maximum mark: 3] | Reference code | 22N.2.HL.TZ0.2 |
| Level | HL | Paper | 2 | Time zone | TZ0 |
| Command term | Explain, Identify | Question number | 2 | Adapted from | N/A |
When performing dihybrid crosses with fruit flies (Drosophila), Morgan discovered that his results did not correspond to the expected Mendelian ratios. He explained this by suggesting that there is an exchange of genetic material between chromosomes. The image shows his diagram for three gene loci on a pair of homologous chromosomes during meiosis.
[Source: Morgan, T.H., 1916. Scheme to illustrate double crossing over. [diagram online] Available at:
https://upload.wikimedia.org/wikipedia/commons/0/0e/Morgan_crossover_1.jpg.]
Identify the stage of meiosis shown where exchange of genetic material occurs.
[1]
prophase 1/ first prophase;
The diagram used in this question came from a paper published by Morgan in 1915 and does not show current understanding of the behaviour of chromosomes in meiosis. Nevertheless, 60% of candidates identified prophase 1 as the stage of meiosis when genetic material is exchanged.
Explain the reason that Morgan’s results did not agree with expected Mendelian ratios in a dihybrid cross.
[2]
- gene linkage/genes located on the same chromosome;
- independent assortment does not occur;
- no recombination unless there is crossing over (between the genes);
Reject sex-linkage.
Many candidates realised that there was gene linkage (not sex-linkage) and that this would prevent independent assortment. Better prepared candidates also understood that recombination also happens with unlinked genes (with 50% of offspring typically being recombinants). Recombination frequency with linked genes is typically lower, as the only way of producing recombinants is through crossing over. The frequency depends on the distance of separate of the gene loci.
