Complementation (genetics)
1728034
203824951
2008-04-06T20:21:45Z
165.230.222.160
/* Example of a Simple Complementation Test */
{{Expand|date=January 2007}}
{{for|complementary DNA generated from [[RNA]] though [[reverse transcription]]|complementary DNA}}
{{otheruses|complementation}}
In [[genetics]], '''complementation''' refers to a relationship between two different [[strain (biology)|strains]] of an organism which both have homozygous recessive [[mutation]]s that produce the same [[phenotype]] (for example, a change in wing structure in flies). These strains are true-breeding for their mutation — when crossed with themselves they will always produce offspring with the mutant phenotype. If, when these strains are crossed with each other, some offspring show recovery of the [[wild-type]] phenotype, these strains show "genetic complementation". A '''complementation test''' (sometimes called a "cis-trans" test) refers to this experiment, developed by [[United States|American]] [[geneticist]] [[Edward B. Lewis]]. It answers the question: "Does a wild-type copy of gene X rescue the function of the mutant allele that is believed to define gene X?". If there is an allele with an observable phenotype whose function can be provided by a wild type genotype (i.e., the allele is recessive) — one can ask whether the function that was lost because of the recessive allele can be provided by another mutant genotype. If not, the two alleles must be defective in the same gene. The beauty of this test is that the trait can serve as a read-out of gene function even without knowledge of what the gene is doing at a molecular level. <ref>[http://ocw.mit.edu/NR/rdonlyres/Biology/7-03Fall-2004/233B3544-E379-44C5-BC31-EF7D864AE73B/0/lecture2.pdf Genetics lectures 1-3 '03<!-- Bot generated title -->]</ref>
Complementation arises because loss of function in [[gene]]s responsible for different steps in the same [[metabolic pathway]] can give rise to the same phenotype. When strains are bred together, offspring inherit wildtype versions of each gene from either parent. Because the mutations are recessive, there is a recovery of function in that pathway, so offspring recover the [[wild-type]] [[phenotype]]. Thus, the test is used to decide if two independently derived [[recessive gene|recessive]] [[mutant]] [[phenotype]]s are caused by mutations in the same [[gene]] or in two different genes. If both parent strains have mutations in the same gene, no normal versions of the gene are inherited by offspring; they express the same [[mutant]] phenotype and complementation has failed to occur.
In other words:
:*If the combination of two [[haploid]] genomes containing different recessive mutations yields a mutant phenotype, then the mutations must be in the same gene.
:*If the combination of two haploid genomes containing different recessive mutations yields the wild type phenotype, then the mutations must be in different genes.
==Example of a Simple Complementation Test==
For a simple example of a complementation test, suppose a geneticist is interested in studying two strains of white-eyed flies of the species [[Drosophila melanogaster]]. In this species, [[wild type]] flies have red eyes and eye color is known to be related to two genes, A and B. Each one of these genes has two alleles, a dominant one that codes for a working protein (''A'' and ''B'' respectively) and a recessive one that codes for a malfunctioning protein (''a'' and ''b'' respectively). Since both proteins are necessary for the synthesis of red pigmentation in the eyes, if a given fly is [[homozygous]] for either ''a'' or ''b'', it will have white eyes.
Knowing this, the geneticist may perform a complementation test on two separately obtained strains of pure-breeding white-eyed flies. The test is performed by crossing two flies, one from each strain. If the resulting progeny have red eyes, the two strains are said to complement; if the progeny have white eyes, they do not.
If the strains complement, we imagine that one strain must have a genotype aa BB and the other AA bb, which when crossed yield the genotype AaBb. In other words, each strain is homozygous for a different deficiency that produces the same phenotype. If the strains do not complement, they both must have genotypes aa BB, AA bb, or aa bb. In other words, they are both homozygous for the same deficiency, which obviously will produce the same phenotype.
==Exceptions==
There are exceptions to these rules. Two non-allelic mutants may occasionally 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 gene|dominant negative]]. Another exception is [[transvection (genetics)|transvection]], in which the heterozygous combination of two alleles with mutations in different parts of the gene complement each other to rescue a wild type phenotype.
==References==
{{Reflist}}
[[Category:Classical genetics]]
[[de:Komplementation]]
[[sr:комплементарност]]