onelocus

Onelocus is a term used in genetics to refer to a trait or phenotype that is determined primarily by a single genetic locus. It describes a situation where variation in the trait is largely attributable to allelic differences at one specific location on the genome, in contrast to polygenic traits that are influenced by many loci. In many contexts, onelocus is used informally to highlight a monogenic basis for a phenotype and to distinguish it from complex traits with multifactorial or multifocal inheritance patterns.

In practice, traits described as onelocus follow patterns consistent with Mendelian inheritance, including dominance, recessivity, or codominance. However, real-world cases can exhibit complexities such as incomplete penetrance, variable expressivity, environmental modifiers, or genetic background effects that modulate the observed phenotype. Genetic mapping approaches such as linkage analysis, family studies, and positional cloning are commonly applied to identify the responsible locus and the causative gene.

The term is often descriptive rather than a formal classification; some traits once thought to be controlled by a single locus may later be found to involve additional loci or genetic heterogeneity. As such, the designation of a trait as onelocus can evolve with new evidence from genetic, functional, or epidemiological studies. In disease genetics, identifying a primary single locus can aid diagnosis and research into pathogenic mechanisms, while acknowledging that modifiers may influence clinical outcome.

See also: locus, single-gene trait, Mendelian inheritance, genetic mapping, monogenic.