dominant allele

Think of two switches wired to the same lamp, one from each parent. A dominant allele is a switch that turns the lamp on even if the other switch is off: a single copy is enough to show its effect. The masked partner is the recessive allele, and the individual carrying one of each looks just like one carrying two dominant copies.

More precisely, an allele is dominant when a heterozygote — someone with one copy of each version of the gene — shows the same trait as a homozygote with two dominant copies. Dominance is a relationship between two alleles, not a property an allele has all by itself; the same allele can be dominant over one variant and recessive to another.

Dominance often arises because one functional copy of a gene makes enough product to do the job, so the loss of the other copy goes unnoticed. But this is not always why, and being dominant says nothing about whether an allele is common, beneficial, or 'stronger' in any moral sense — many harmful conditions are dominant and many helpful alleles are recessive.