Some traits appear whenever a particular allele is present. A dominant trait is expressed in the phenotype even if only one copy of the dominant allele is inherited. The corresponding recessive allele remains hidden in heterozygotes.

Classic examples include the ability to taste certain compounds, free-hanging earlobes in some populations, and numerous Mendelian disorders such as Huntington’s disease. In a simple dominant-recessive pair, heterozygous and homozygous dominant individuals show the same phenotype, although dosage effects sometimes produce subtle differences.

Pedigree analysis reveals dominant transmission when the trait appears in successive generations and is transmitted by affected individuals to roughly half their offspring. Molecular testing confirms the causative allele. Dominance is a relationship between alleles, not an intrinsic property of a gene.

• Expressed with only one copy of the allele
• Masks the effect of a recessive allele
• Visible in heterozygotes
• Transmitted to approximately half of offspring