1. (in genetics) Describing the allele that is expressed in the phenotype when two different alleles of a gene are present in the cells of an organism. For example, the height of garden peas is controlled by two alleles, ‘tall’ (T) and ‘dwarf’ (t). When both are present (Tt), i.e. when the cells are heterozygous, the plant is tall since T is dominant and t is recessive. Various mechanisms can cause one allele to exert dominance over another in determining phenotype. For example, the recessive allele may simply be defective in some way, and unable to produce any protein product, or at least sufficient amounts of product, so its effects are only apparent when both alleles are recessive (i.e. homozygous recessive). Alternatively, the dominant allele may be aberrant in some way, and hamper the effects of its normal recessive counterpart, e.g. by blocking a metabolic pathway. Another possibility is that the dominant allele codes for a modified protein that evades normal control mechanisms and overrides the normal, recessive allele—a so-called gain-of-function mutation. See also codominance; incomplete dominance.

2. (in ecology) Describing the most conspicuously abundant and characteristic species in a community. The term is usually used of a plant species in plant ecology; for example, pine trees in a pine forest.

3. (in animal behaviour) Describing an animal that is allowed priority in access to food, mates, etc., by others of its species because of its success in previous aggressive encounters. Less dominant animals frequently show appeasement behaviour towards a more dominant individual, so overt aggression is minimized. In a stable group there may be a linear dominance hierarchy or peck order (so called because it was first observed in domestic fowl), with each animal being subservient to those above it in the hierarchy and taking precedence over those below it.