Heterokaryons can be used for complementation tests. An example of incomplete dominance is flower color in snapdragons. The heterozygote expresses aspects of the phenotypes of each of the homozygotes. An example of codominance is the ABO blood group system in humans. Genotypic and phenotypic ratios identical to incomplete dominance.
Some alleles act as recessive lethals.
In such cases homozygous recessive individuals may be conceived, but typically are not born. In some other examples the recessive phenotype is often less viable than the dominant phenotype, this is referred to as semi-lethal. If an allele of one gene masks the expression of another gene it is referred to as epistasis. Epistasis is inferred when an allele of one gene eliminates expression of alleles of another gene and expresses its own phenotype instead. It points to interaction in genes in some biochemical or developmental sequence.