PRESENTED BY- SOMASHREE DAS

REG NO- 15KUST4023

M.Sc 3rd SEM

DEPARTMENT OF BIOTECHNOLOGY

RAMAIAH COLLEGE OF ARTS, SCIENCE AND COMMERCE 1/12

Complementation occurs when two strains of an organism with different homozygous recessive mutations that produce the same mutant phenotype (for example, a change in wing structure in flies) produce offspring with the wild-typephenotype when mated or crossed.

Complementation will occur only if the mutations are in different genes.

In this case, each strain's genome supplies the wild-type allele to "complement" the mutated allele of the other strain's genome. Since the mutations are recessive, the offspring will display the wild-type phenotype.

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A complementation test (sometimes called a "cis-trans" test) can be used to test whether the mutations in two strains are in different genes.

Complementation will not occur if the mutations are in the same gene. The complementation test was developed by American geneticist Edward B.

Lewis. A heterozygote with two mutations of the same gene will produce only

mutant mRNAs, which result in mutant enzymes. The two mutations will complement each other and produce the wild-type. Mutations that fail to each other are termed as functional alleles. The test for defining alleles strictly on the basis of functionality is termed

the cis-trans complementation test.

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Thus the two mutations will complement each other and produce the wild-type. Mutations that fail to complement each other are termed as functional alleles. The test for defining alleles strictly on this basis of functionality is termed as cis-

trans complementation test.Source: DNA, it’s mutation, repair and damage; pg. no. 319; Genetics-Tamarin

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DEPARTMENT OF BIOTECHNOLOGY 4/12

A heterozygote of two recessive mutations can have either trans or cisarrangement.

In trans position, functional alleles produce a mutant phenotype.

In cis position, functional alleles produce a wild-type phenotype.

This difference in phenotype is called cis-trans position effect.

Source: DNA, it’s mutation, repair and damage; pg. no. 319; Genetics-Tamarin

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DEPARTMENT OF BIOTECHNOLOGY 5/12

The complementation test is used to establish how many units of genes are defined by a given test of mutations that express the same mutant phenotypes.

In Benzer’s work with rII mutants, the non-permissive strain K12(lamda) was infected with a pair of rII mutant phages.

If the phages produce progeny, the two mutants are said to complement each other, meaning that the two mutations must be in different genes that encode different products.

If no progeny phages are produced, the mutants are not complementary indicating that mutations are in the same functional unit.

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Source: Mutation; pg.no.407; Genetics-Peter.J.RusselRAMAIAH COLLEGE OF ARTS, SCIENCE AND COMMERCE

DEPARTMENT OF BIOTECHNOLOGY 7/12

Each rII mutant phage that co-infects the non-permissive E.coli strain K12(lamda) carries one rII mutation, a configuration of mutations called the trans configuration.

When both the mutations are carried on same chromosome, the configuration is called cis configuration of mutations.

Benzer called the genetic unit of function revealed by the cis-trans test as cistron. A cistron is the smallest segment of DNA that encodes a piece of RNA.

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Example of complementation in a diploid organism :-

Two true-breeding mutant strains of Drosophila melanogaster have black body color instead of wild-type grey-yellow.

When the two strains are crossed, all the F1 flies have wild-type body color.

This is because complementation has occurred between mutations intwo genes, each of which is involved in the body color phenotype.

That is a recessive autosomal gene, ebony(e), when homozygous, produces a black body color.

On another autosome a different recessive gene, black(b), also produces a black body color when homozygous.

Because the two parents are homozygotes, they are genotypically e/eb+/b+ and e+/e+b/b, which is equivalent to trans configuration.

The F1 have wild-type body color because complementation has occurred.

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Source: Mutation; pg.no. 408; Genetics-Peter.J.Russel

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DEPARTMENT OF BIOTECHNOLOGY 10/12

The number of units of functions (genes) is determined by complementation tests.

Given a set of mutations expressing the same mutant phenotype, two mutants are combined and the phenotype is determined.

If the phenotype is wild-type, the two mutations have complemented and must be in different units of function.

If the phenotype is mutant, the two mutations have not complemented and must be in the same unit of function.

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Genetics- Peter. J. Russel Genetics- Tamarin www.wikipedia.org www.britanica.com

THANK YOU…………..

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DEPARTMENT OF BIOTECHNOLOGY 12/12