A dominant gene is a variant of a gene that is expressed more strongly than another variant of the same gene, known as a recessive gene). An individual receives two versions of each gene, known as alleles, from each parent. If the alleles of a gene are different, one allele will be expressed, and it is the dominant gene. The effect of the other allele, called recessive, is masked. Dominance is not inherent to an allele or its traits (phenotype). It is a strictly relative effect between two alleles of a given gene of any function; one allele can be dominant over a second allele of the same gene, recessive to a third, and co-dominant with a fourth).

Dominance differs from epistasis, which is the phenomenon of an allele of one gene masking the effect of alleles of a different gene). Dominant alleles show their effect even if the individual only has one copy of the allele (also known as being heterozygous), while recessive alleles only show their effect if the individual has two copies of the allele (also known as being homozygous).

Examples of dominant and recessive genes include eye color and blood groups. The allele for brown eyes is dominant over the allele for blue eyes, so if an individual has one allele for brown eyes and one allele for blue eyes, their eyes will be brown. However, if both alleles are for the recessive trait (in this case, blue eyes), the individual will inherit blue eyes.