Definition

A dominant trait shows up whenever at least one dominant allele is present, since that single copy is enough to mask a paired recessive allele. Because both the homozygous dominant ($PP$) and heterozygous ($Pp$) genotypes display the same dominant phenotype, you cannot tell them apart by appearance alone, which is why a purple-flowered pea plant can still carry and pass on a hidden white allele. Mechanistically, dominance describes a relationship between two alleles with respect to a specific phenotype, where the heterozygote matches the phenotype of the homozygote for that allele; complete dominance often arises when one functional gene copy makes enough protein for a normal outcome (haplosufficiency), while dominant disease alleles typically act through gain of function, dominant-negative interference, or haploinsufficiency, and the same allele can be dominant for one phenotype yet recessive for another.