complementer

A complementer is a gene or genetic element that can restore normal function to a organism or cell carrying a defect by providing a functional product in trans. In genetic research, the concept is central to complementation tests, where two mutants with recessive alleles in different genes may display a wild-type phenotype in the offspring because each mutant allele is complemented by the normal allele from the other parent. The gene that supplies the missing function in this context is referred to as the complementer.

Complementation tests are used to determine whether mutations lie in the same gene (they do not complement) or in different genes (they do complement). In practical work, researchers perform plasmid- or chromosome-based complementation assays by introducing a functional copy of a gene into a mutant strain; restoration of the wild-type phenotype indicates that the introduced gene acts as a complementer to the mutation.

The concept extends to the notion of complementation groups, which are sets of mutations that fail to complement each other and are inferred to affect the same gene or functional unit. Such groups help in mapping gene function and understanding genetic pathways.

Limitations exist: results can be influenced by genetic background, gene dosage, regulatory elements, or epistatic interactions that obscure true complementation. Partial or conditional complementation can occur if expression levels or tissue-specific regulation affect the ability of a gene to compensate for a defect.

Outside genetics, the term complementer is less standardized, occasionally used more loosely to describe a component that adds missing functionality to a system. In formal use, it remains most precisely defined within genetic complementation.