In Genetics, What Is Complementation?

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  • Written By: T. Carrier
  • Edited By: John Allen
  • Last Modified Date: 07 November 2019
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The study of cellular changes, or mutations, is a prime research area in genetics. One area of interest to scientists concerns how an organism can overcome genetic abnormalities to retain the normalized version of a trait. Complementation tests can address this issue by studying organisms with genetic mutations and their offspring. If an offspring trait is expressed normally even in the presence of known genetic mutations, then the genetic relationship is said to be complementary.

In order to understand complementation, certain genetic terms must first be defined. Gene expressions are at the most basic level of complementation, and genes are small units within an organism that hold and pass on traits. They are comprised of deoxyribonucleic acid (DNA) and ribonucleic acid (RNA). The different forms of a gene are known as alleles, and each gene is stored on DNA structures called chromosomes. A gene may be expressed in the form of letter symbols called genotypes, but its actual physical expression in the form of a trait — like blue eyes — is called a phenotype.

Since most organisms have two chromosome sets, each trait will generally have a gene on each chromosome. When the alleles of the two genes are the same, the expression is referenced as homozygous. If, however, the gene alleles on the chromosomes are different, a heterozygous expression results.


Scientists consider complementation when studying genetic mutations. These abnormalities arise when a transformation takes place within the DNA. They can occur due to environmental factors or by cellular errors or processes. Mutations tend to be recessive — or less common and influential — rather than dominant. When a phenotype, or physical expression, is part of the normal average rather than a mutated expression, it is known as a wild-type phenotype.

Complementation results when a cell or organism has a normal genetic expression even if it is the product of two known mutations. For example, in the fruit fly species, most flies have red eyes. Mutants, however, have white eyes. If the offspring of two white-eyed fruit flies possesses red eyes, then the offspring likely had complementary genetic features.

Such conclusions can be drawn because mutations are recessive traits that require two different recessive contributions. If a mutation takes place on different genes, then the dominant versions of one organism’s gene could supersede the recessive version of the second organism’s gene. This produces a normal phenotype.

Genetic researchers find usefulness in complementation tests because complement DNA and complement sequence can help determine where specific mutations arise on a gene, and which genes are responsible for the mutation. These tests combine two cells with the same expressed mutation together via a complement receptor. As outlined, scientists then study if the cells produce an offspring that also has the mutation in question. A non-complementation finding alerts researchers that the mutation likely occurs on the same gene in both organisms. If, however, the offspring has abolished the mutation, then the abnormality most likely occurred on two different genes.


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