The system of sex determination in chimpanzees (Pan troglodytes) is the same as in other mammals. A chimpanzee has 48 chromosomes in the nuclei of its body cells.

What can be deduced from this information?

A. The sex of the chimpanzee

B. The number of genes in each chromosome

C. Whether non-disjunction has occurred

D. The number of autosomes in a diploid cell

D

There were claims by some teachers that both C and D could be correct answers. The instruction to students should always be to choose the best of the available answers. This necessitates reading all four answers and not stopping after reaching an answer that on the surface looks correct. Here answer D is undoubtedly correct – if we know that chimpanzees have two sex chromosomes and that there are 48 chromosomes in the nuclei of a chimpanzee’s body cells, we can deduce the number of autosomes. 40 % of candidates chose this answer and the high discrimination index shows that it was favoured by the stronger candidates. Answer C is much less convincing than D – the stage at which non-disjunction might or might not have occurred isn’t specified so the deduction would be vague. Also, non-disjunction is diagnosed by looking at the chromosome types, not just the overall chromosome number.