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.