5.1 Introduction

Learning Objectives

  • Recall that mutant screening is one of the first steps geneticists use to investigate biological processes.
  • Distinguish between allelic and non-allelic mutations.
  • Describe the use of the Complementation Test (otherwise known as cis-trans test) and identify complementation groups.


A particular phenotype is usually the result of the biochemical product(s) of multiple genes acting in a pathway. Polygenic inheritance occurs when one characteristic is controlled by two or more genes. Often, the genes are large in quantity but small in effect. Examples of human polygenic inheritance are height, skin colour, eye colour, and weight. A mutation in any one given gene of the set governing a phenotype, can result in an alteration of the manifested trait. How then do geneticists determine if two mutants which have the same phenotype carry their mutation in the same gene or in different genes? We achieve this by the use of the Complementation Test.

A complementation test consists of classical Mendelian genetic crosses to determine if one mutant can complement another or, in other words, produce the wild type phenotype. More recently, transformation of DNA with a gene has been used to determine if inserting a single gene into a cell/organism can rescue a mutant phenotype.

Image showing fruit flies with white eyes and results of crosses between different strains to produce red eyed flies
Figure 5.1.1 Example of Genetic Complementation. Two strains of flies are white eyed because of two different autosomal recessive mutations which interrupt a single pigment-producing metabolic pathway at two different points. Flies from Strain 1 have complementary mutations to flies from Strain 2 because when they are crossed, the offspring are able to complete the full metabolic pathway and, thus, have red eyes. A fly from Strain 1 is not complementary to another fly from Strain 1 because they are both homozygous for mutations affecting the same point in the pathway, so their offspring will be white-eyed.

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Introduction to Genetics Copyright © 2023 by Natasha Ramroop Singh, Thompson Rivers University is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International License, except where otherwise noted.

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