What is Dominance?
The question of dominance has been the focus of debate since Sewall Wright and Ronald Fisher introduced the issue in the late 1920s.44-47 At the molecular level, dominance of an allele is likely to involve the presence of a biochemical function encoded by a housekeeping gene, while recessive alleles encode nonfunctional products or none at all.48 Of target genes, only those that occupy the rate-limiting step of a biochemical pathway would produce a semi-dominant effect on the phenotype. The expression of the remaining target genes in a pathway would reflect the action of the regulatory system, whose components would exhibit a dosage effect and thus show semi-dominant behavior in heterozygotes. The anthocyanin pigment pathway genes in maize provide an example. Most of the structural genes of the biochemical pathway have been identified.49 Functional alleles show complete dominance over null alleles for five of the six known steps. The one exception is the c2 gene that encodes the rate-l
