Question 22N.2.SL.TZ0.4
| Date | November 2022 | Marks available | [Maximum mark: 6] | Reference code | 22N.2.SL.TZ0.4 |
| Level | SL | Paper | 2 | Time zone | TZ0 |
| Command term | Calculate, Draw, Explain | Question number | 4 | Adapted from | N/A |
Thrombophilia is a human genetic condition where the blood has an increased tendency to clot. The condition is caused by a single base substitution mutation in DNA. If a person is homozygous for the gene, they are at greater risk for developing a blood clot than an individual who is heterozygous. The pedigree chart shows the inheritance of thrombophilia in a family.
[Source: From The New England Journal of Medicine, Hopmeier, P. and Krugluger, W., n.d. Factor V Leiden and
Thrombophilia. [diagram online] Available at: https:// www.nejm.org/doi/full/10.1056/nejm199505183322014
[Accessed 2 November 2021], Vol. 332 No. 20. Page No. 1381. Copyright © 1995 Massachusetts Medical
Society. Reprinted with permission from Massachusetts Medical Society.]
Draw the symbol for individual X on the diagram.
[1]
The majority of candidates managed to draw the correct symbol for a heterozygous female in the correct place in (a). However, there were a few who thought that X was male. Just over a half correctly gave 50% for (b). There were a large number of very pleasing answers in (c) to explain why the gene is not sex linked including that if it were sex linked, then you could not have a heterozygous male, and if it were sex linked, the daughter would have been heterozygous. The explanations in (d ) were not of such a high quality, with a large proportion failing to note that it hinges on the coding for a different amino acid, thus producing a different protein. Students should be careful about restating the stem as the 'protein changes' did not score as that was in the stem.
Calculate the probability of male Y having an allele for the disorder.
[1]
50%/ 0.5/ 1/2;
Explain how the information in the box labelled B indicates that the gene is not sex-linked.
[2]
- if it was sex-linked it would be on the x chromosome;
- there cannot be a heterozygous male if the trait is sex-linked
- males would pass the allele to their daughter;
- daughter is not shown as heterozygous so it is not sex-linked;
Explain how a single base substitution mutation in DNA can cause a change to a protein.
[2]
- sequence of DNA bases determines the amino acid sequence of a protein;
- changing one base (on the DNA) can cause the triplet /mRNA to code for a different amino acid;
- changing one base (on the DNA) causes a different protein to be made (during translation);