Modifiers, suppressors and pleiotropic genes

Lethality and Interaction of Genes
Interaction of genes
Abbreviated genotypic ratio
Two gene pairs affecting same character
Complementary genes
Duplicate genes
Additional interactions involving two gene pairs
Interactions between more than two gene pairs
Modifiers, suppressors and pleiotropic genes
Meiotic drive, segregation distortion and selfish genes
Penetrance and expressivity

Although we normally ascribe the control of phenotype pertaining to a specific character to one particular gene, it is normally influenced in various ways by several genes. Although some of these influences have been characterized in terms of complementary genes, epistatic genes, duplicate genes, etc., in several other cases, the influence can not be characterized in such definite terms. Such genes, which modify effect of other genes without any characteristic form, are sometimes described as modifiers. These modifiers normally change the phenotypic effect of other genes in quantitative manner. Many genes responsible for dilution of body colour may belong to this category. There may always be present modifiers with minor effects on quantitative characters like yield or height, etc. but these genes can not be easily identified.

There are also modifiers, which will not allow mutant allele of another gene to express either fully or partially. These modifiers have been called suppressors and result in wild phenotype, so that one will have to determine whether it is due to reversion of mutant to wild type or due to a suppressor. Some of the examples of suppressor genes are su-h (suppressor for hairy wings) and su-S (suppressor for star eye shape). Mechanism of action of such suppressor genes will be discussed in The Genetic Code on Genetic Code.

Modifiers may also influence the degree of dominance expressed by another gene. In Abraxas grossulariata, a moth, it could be possible, to establish lines with same gene (lutea - yellow colour) dominant in one case and recessive in the other. This was proved to be due to the effect of modifiers (Ford, 1940).

There are certain other genes, which in addition to their main effect, may also act as modifiers for another but entirely different gene. Such genes having more than one effect are called pleiotropic genes. Some examples of pleiotropic genes are as follows :

  1. Some genes for bristles, eye and wing may influence the number of facets in bar-eyed individuals.
  2. Gene for white eyes may affect shape of sperm-storage organs in females and also some other structures.
It is believed that pleiotropic genes must be more common than earlier suspected, because indirectly every gene may be involved in the expression of more than one traits. Thus most genes, if not all, affect more than one phenotypic traits and are thus pleiotropic in nature.