How does a dominant allele affect an inherited trait?

In genetics, a dominant allele is a version of a gene that exerts a greater influence on an inherited trait than its counterpart, known as a recessive allele. When an organism is heterozygous—meaning it carries two different alleles for a specific trait—the dominant allele determines the phenotype, or the observable characteristic, by masking the expression of the recessive allele.1234

The degree of this influence can vary:

• Complete Dominance: The dominant allele completely masks the presence of the recessive allele. In this case, the phenotype of a heterozygous individual is indistinguishable from that of an individual who is homozygous for the dominant allele.23
• Incomplete Dominance: The dominant allele has a greater influence than the recessive one but does not fully mask it. The resulting phenotype is often an intermediate expression of the two traits.23

It is important to note that not all alleles follow these patterns; some are codominant, where both alleles contribute equally to the phenotype, as seen in the human ABO blood group system.1