Question 21M.2.SL.TZ1.a
| Date | May 2021 | Marks available | [Maximum mark: 4] | Reference code | 21M.2.SL.TZ1.a |
| Level | SL | Paper | 2 | Time zone | TZ1 |
| Command term | Outline | Question number | a | Adapted from | N/A |
Outline the inheritance of blood types in the ABO blood system in humans.
[4]
a. i and IA and IB are alleles (of the blood group gene) ✔
b. IA is dominant and i is recessive / IB is dominant and i is recessive ✔
c. Group O (only) with ii and Group A with IAi or IAIA / Group B with IBi or IBIB
d. IA and IB are co-dominant so Group AB with IAIB ✔
e. one allele/copy of the gene inherited from each parent/Punnett square showing this ✔
Disallow mpa if allele notation is incorrect (such as using different letters for different alleles) but allow other mps.
The notation Ii can be accepted instead of i.
Reject blood groups or types being dominant/recessive – it should be alleles.
For a well-prepared candidate this was a very routine exercise, but many candidates were not so prepared. A common fault was to confuse blood groups with the alleles that determine the groups, either in notation for alleles, or overall. Commonly, answers did not distinguish between gene and allele. Some candidates focused too much on the consequences of blood groups (including rhesus +ve/−ve) for blood transfusion which was not relevant to the question.
