People who are homozygous dominant for the West Nile resistance trait have the strongest resistance to West Nile Virus. Explain why these people do NOT have a heterozygous advantage, and compare and contrast this with an example we discussed in class. Solution Let us suppose \'RR\' genotype is homozygous dominant for West Nile resistance trait. \'Rr\' genotype is heterozygous. This may be an example of codominance. Alleles that lack dominant and recessive relationship and are both observed phenotypically to same degree are called codominant. So, the phenotypic effect of each allele is observable in the heterozygous condition. The heterozygous condition gives rise to a phenotype different from either of the homozygous genotype. For example, M-N blood group in humans. In this case, when both alleles are present, effect is high..