Tutorial Answer 6

(c) and (d) are correct because, in different ways they say the same thing. (c) describes the phenotypic condition typical of incomplete dominance. In incomplete dominance the heterozygote is phenotypically (in this case described as a series of clinical conditions) distinguishable from both homozygotes. (d) describes the genetic conditions of incomplete dominance, in that neither gene can be said to be mask the effects of the other. In complete dominance in the Mendelian sense, the effects of the dominant gene completely mask those of the non-dominant, or recessive gene in the heterozygote. Thus, for example, if the gene for normal hemoglobin were dominant over the gene for sickle cell hemoglobin, the heterozygote, HbAHbS would be completely normal and show no sickling even under extreme oxygen deprivation. Thus, the homozygous dominant genotype, HbAHbA, would be phenotypically indistinguishable from the heterozygote.