Modifiers

A modifier gene is a gene that can alter the phenotypic effect of another gene but which does not produce a phenotype in and of itself. There are two general types that we observe most commonly:

• Enhancer: An enhancer gene (not to be confused with enhancer elements at the molecular level!) is a gene that has an allele which enhances the phenotypic effect of another gene. For example, let us say that there is a gene (we will call it the D gene) that affects coat color in horses and that there are two alleles; the D allele which is associated with black coat color and d which is associated with brown coat color. If a second gene, E, also has two alleles, E and e and we observe the following phenotypes:
  
  
  • D_E_ Dark Black
  
  
  • D_ee Gray (Light Black)
  
  
  • ddE_ Dark Brown
  
  
  • ddee Light Brown
  
  
  Then we would classify the E gene as an enhancer of D. More specifically, it is the E allele that is the enhancer: the presence of the E allele does not give rise to a novel phenotype, rather the allele enhances the effect of the D gene by making the pigment more concentrated, and thus darkening the coat color. Enhancement can come about in many ways, this is just one example, but the common idea is that the enhancing gene does not have any phenotype associated with it, it simply alters the phenotype produced by another gene by enhancing the effect.


• Suppressor: As with an enhancer, a suppressor gene does not produce a phenotype itself, it simply weakens, or eliminates, the phenotypic effect of another gene. Also, as with an enhancer, it is a specific allele of the gene that suppresses the effect of the other gene. As a hypothetical example, we can consider a gene, call it gene B, in rabbits that has two alleles; the B allele is associated with a brown coat and the b allele is associated with a white coat. If there is a second gene, the C gene, with two alleles such that we observe the genotype/phenotype relationships:
  
  
  • B_C_ Brown
  
  
  • B_cc Brown
  
  
  • bbC_ White
  
  
  • bbcc Brown
  
  
  Then we see that the cc genotype is not producing an effect of its own, but it suppresses the bb phenotype, which is typically white, so that a Brown coat is observed instead. In this case we say that the c allele is a suppressor of the B gene, in particular the b allele. The key, once again, is that c is not producing a novel phenotype, it is just modifying the phenotype produced by bb. (Notice the important fact that bbcc has the phenotype of the BB genotype. This is why we say that c is a suppressor. If bbcc had some other phenotype - say a gray coat - then c would not be suppressing bb. Instead, in such a situation it would have a unique phenotype associated with it.)