Mendel's First Law

Variations to Mendel's First Law

Pedigree Analysis

Mendel's Second Law

Chi-Square Test



Modifier Genes

Penetrance and Expressivity

Study Questions

Mendelian Genetics Overheads

Mendelian Genetics WWW Links

Genetic Topics

Modifier Genes

Instead of masking the effects of another gene, a gene can modify the expression of a second gene. In mice, coat color is controlled by the B gene. The B allele conditions black coat color and is dominant to the b allele that produces a brown coat. The intensity of the color, either black or brown is controlled by another gene, the D gene. At this gene, the dominant D allele controls full color whereas the recessive d allele conditions a dilute or faded expression of the color expression at the B gene. Therefore, if a cross is made among mice that are BdDd, the following phenotypic distribution will be seen:

  • 9 B_D_ (black)
  • 3 B_dd (dilute black)
  • 3 bbD_ (brown)
  • 1 bbdd (dilute brown)

The D gene does not mask the effect of the B gene, rather it modifies its expression.

Modifier genes - genes that have small quantitative effects on the level of expression of another gene

Copyright © 2000. Phillip McClean