Complementation test, also known as a genetic complementation test or a cis-trans test, is a technique used in genetics to determine whether two different mutations (alleles) that produce the same phenotype are located in the same gene (allelic) or in different genes (non-allelic, or in different loci).

Performing a Complementation Test

Objective:

To determine if two mutations that produce the same mutant phenotype are located in the same gene (allelic) or in different genes (non-allelic).

Steps:

  1. Crossing Individuals with Mutant Phenotypes:
    • Select two individuals, each carrying a different recessive mutation that results in the same mutant phenotype (e.g., both show a non-functional enzyme activity).
  2. Generating the Crosses:
    • Cross these individuals to produce a hybrid offspring (heterozygotes) that carries both mutant alleles.
  3. Observing the Phenotypes:
    • Examine the phenotype of the hybrid offspring (heterozygotes).
    • If the hybrid shows the wild-type (normal) phenotype, it indicates that the mutations are likely in different genes (non-allelic). This is because each allele can complement the other, providing the functional enzyme needed for normal phenotype.
    • If the hybrid shows the mutant phenotype, it indicates that the mutations are likely in the same gene (allelic). This is because both alleles are unable to produce the functional enzyme independently.
  4. Interpretation:
    • Complementation: If the hybrid offspring shows the wild-type phenotype, the mutations complement each other, suggesting that they are in different genes.
    • Non-complementation: If the hybrid offspring shows the mutant phenotype, the mutations do not complement each other, suggesting that they are in the same gene.

Example of Complementation Test

Scenario: In fruit flies (Drosophila melanogaster), two different recessive mutations, A and B, both result in white eye color instead of the wild-type red eye color.

  1. Crossing Individuals:
    • Cross a fly with genotype A/A (homozygous for mutation A, white eyes) with a fly with genotype B/B (homozygous for mutation B, white eyes).
  2. Generating the Crosses:
    • The cross produces offspring with genotype A/B (heterozygous for both mutations).
  3. Observing the Phenotypes:
    • If the A/B heterozygous offspring show red eye color (wild-type phenotype), it indicates that mutations A and B complement each other, suggesting they are in different genes (non-allelic).
    • If the A/B heterozygous offspring show white eye color (mutant phenotype), it indicates that mutations A and B do not complement each other, suggesting they are in the same gene (allelic).

Definitions

  • Homozygous Alleles (Homoalleles): Two identical alleles of a gene present at the same locus on homologous chromosomes. In a homozygous state, both alleles are the same (e.g., AA or bb).
  • Heterozygous Alleles (Heteroalleles): Two different alleles of a gene present at the same locus on homologous chromosomes. In a heterozygous state, one allele is different from the other (e.g., Aa or Bb).

In summary, the complementation test is a valuable tool to determine whether mutations affecting the same phenotype are located in the same gene or in different genes, providing insights into the genetic basis of traits and gene interactions.