Tutorial Answer 7

(a) is correct because, in genetic terms "carriers" are heterozygotes who have one normal and one mutant gene, each of which they will pass on, on average, to half of their offspring. In complete dominance the carrier cannot be distinguished phenotypically from the homozygous normal. In incomplete dominance, the carrier is usually distinguishable in some way from the normal (as in sickle cell trait, where the carrier's cells will sickle under very low oxygen tension, whereas homozygous normal individuals will never show sickling).