In a person who is heterozygous for sickle-cell anemia, where is the mutation found?

A. In every gamete produced

B. Only in gametes carrying an X chromosome

C. In all brain cells

D. In blood plasma

C

This question proved to be too hard for all candidates. Most candidates answered that the mutation for sickle-cell anemia is found in blood plasma. This is probably because they know anemia is related to blood, but failed to realize that plasma has no cells, therefore no DNA (6.2.6 in guide). Many candidates also went for gametes with X chromosome; probably wrongly believing this is a sex-linked disease. Others believed every gamete has the mutation, forgetting that gametes only have half the genetic information, so in a heterozygous individual, not all gametes receive the mutation. All nucleated cells in the body (except half of the gametes) have the mutation; therefore the only possible answer was brain cells.