The genotypes of the offspring from heterozygote intercrosses exhibited a Mendelian segregation at E18.5. The heterozygote expressed none of the disadvantages of homozygotes, yet gained improved viability. Incomplete dominance is the condition in which neither allele dominates the other in one heterozygote. Thus you can tell that each allele is present in the heterozygote. X-linked dominant inheritance will show the same phenotype as a heterozygote and homozygote. The specific case of heterozygote advantage due to a single locus is known as overdominance. Merle is actually a heterozygote of an incompletely dominant gene. However, heterozygote advantage is not the only way a polymorphism can be maintained. The shows how a heterozygote advantage can affect trait evolution. Alternatively, a heterozygote for gene "R" is assumed to be "Rr". This is an example of underdominance (heterozygote disadvantage) leading to disruptive selection. These authors found the heterozygote state was indistinguishable from the noncarrier state. Notice that the frequency of the heterozygote declines in proportion to f. Cases of heterozygote advantage have been demonstrated in several organisms, including humans. The two major and most studied are heterozygote advantage and frequency-dependent selection.