What does the term 'dominant' refer to in genetics?

The term 'dominant' in genetics refers to a type of allele or gene that is always expressed in the phenotype, even when only one copy is present in the genotype. This means that if an individual carries at least one dominant allele for a particular trait, that trait will be visibly expressed, overshadowing any influence from a recessive allele.

For example, consider a trait governed by two alleles: one dominant (A) and one recessive (a). The presence of the dominant allele (A) will result in the dominant trait being exhibited, irrespective of whether the genotype is homozygous dominant (AA) or heterozygous (Aa). In contrast, the recessive trait will only manifest phenotypically when an individual possesses two copies of the recessive allele (aa).

The other choices do not accurately capture the definition of 'dominant' in this context. A gene that is rarely expressed would not be considered dominant, as dominant genes are characterized by their consistent expression. A gene that can mutate does not pertain to dominance or recessiveness; mutation is a separate aspect of genetics. Lastly, the term 'recessive' directly opposes the concept of dominance, as recessive alleles require two copies to be