Which trait is defined as one that always appears when an individual possesses an allele for it?

The trait defined as one that always appears when an individual possesses an allele for it is known as a dominant trait. This characteristic is fundamentally based on how alleles interact during the expression of traits. In genetic terms, if an organism has at least one copy of a dominant allele, the trait associated with that allele will be expressed in its phenotype, which refers to the observable physical and functional characteristics of an organism.

This is in contrast to other types of traits. Recessive traits require two copies of their allele to be expressed, meaning that if there is only one dominant allele present, the recessive trait will not be observed in the phenotype. Codominant traits result in both alleles being fully expressed in a heterozygous organism, while incomplete dominance produces a blend of the two traits. Understanding the difference between these types of interactions helps clarify the role of dominant traits in genetics.