An allele is dominant if the phenotype of the heterozygote, looks like the homozygote of that allele; the other allele in the heterozygote is called recessive.

In humans, for example, the allele for brown eyes (B) is dominant over the allele for blue eyes (b). Thus a person with both alleles, i.e. a Bb genotype, will have brown eyes.

The dominance effect was explained by Mendel, who first used capitals to denote dominant alleles.

An example of dominance is the Hawaiian happy-face spider (Theridion grallator), pictured opposite. The gene for the patterned form is dominant, and the plain yellow form is recessive. Even if just one of the genes at this locus codes for a patterned form, the spider is patterned.