Overdominance the heterozygote has higher fitness than the fitnesses of the two homozygotes
Underdominance, in which the heterozygote has lower fitness than both homozygotes.
Directional selection leads to fixation of the favored allele and elimination of the other allele, as long as no other evolutionary forces act on the population.
#Section 3: Genetics, Cellular and Molecular Biology#Natural selection#Part B Pointers
