A complementation test (sometimes called a "cis-trans" test) is used in genetics to decide if two recessive mutant phenotypes are determined by mutations in the same gene or two different genes. Genetics (from the Greek genno γεννώ= give birth) is the science of genes, heredity, and the variation of organisms. ... In genetics, the term recessive gene refers to an allele that causes a phenotype (visible or detectable characteristic) that is only seen in a homozygous genotype (an organism that has two copies of the same allele). ... This article is concerns biological mutants; for fictional aspects see Mutant (fictional) A mutant is an individual, organism, or new genetic character arising or resulting from an instance of mutation, which is a sudden structural change within the DNA of a gene or chromosome of an organism resulting in the... The phenotype of an individual organism is either its total physical appearance and constitution, or a specific manifestation of a trait, such as size or eye color, that varies between individuals. ... This stylistic schematic diagram shows a gene in relation to the double helix structure of DNA and to a chromosome (right). ...

The American geneticist Edward B. Lewis is credited with developing the test. Genetics (from the Greek genno γεννώ= give birth) is the science of genes, heredity, and the variation of organisms. ... Edward B. Lewis (May 20, 1918–July 21, 2004) was an American geneticist, the winner of the 1995 Nobel Prize in Medicine. ...

True breeding mutants are crossed bringing together the mutant genotypes in an F1 individual. If that individual is mutant, then the complementation has failed, and thus the two alleles are the same gene. If no mutant phenotype is observed in the F1 individual, then the mutant alleles are complemented and must be different genes. Mendelian inheritance (or Mendelian genetics or Mendelism) is a set of primary tenets that underlie much of genetics developed by Gregor Mendel in the latter part of the 19th century. ... The genotype is the specific genetic makeup (the specific genome) of an individual, usually in the form of DNA. It codes for the phenotype of that individual. ... An allele is any one of a number of alternative forms of the same gene occupying a given locus (position) on a chromosome. ...

That is:

• If the combination of two haploid genomes containing different recessive mutations yields a mutant phenotype, then the mutations must be in the same gene (alleles).
• If the combination of two haploid genomes containing different recessive mutations yields the wild type phenotype, then the mutations must be in different genes.

There is an exception to this rule. Two non-allelic mutants may fail to complement (called "non-allelic non-complementation" or "unlinked non-complementation"). This situation is rare and is dependent on the particular nature of the mutants being tested. For example, two mutations may be synthetically dominant negative. Haploid (meaning simple in Greek) cells have only one copy of each chromosome. ... In genetics, the term dominant gene refers to the an allele that causes a phenotype that is seen in a heterozygous genotype. ...