Single genes with multiple effects:

Most genes actually have multiple effects throughout a cell and/or throughout a multicellular organism.
The multiple effects of a single gene on the phenotype of an organism is called pleiotropy It can occur due to the following (86)
1. Expression of a single gene that can affect cell function in more than 1 way.
2. A gene may be expressed in different cell types in a multicellular organism.
3. A gene may be expressed at different stages of development.

Gene interactions (section 4.2):

Essentially all traits are affected by the contributions of many genes (86).
The phenomenon of how the allelic variants of 2 different genes affect a single trait is called gene interaction (86).

A Cross Can Produce 4 Distinct Phenotypes

Epistasis is when the alleles of one gene mask the phenotypic effects of the alleles of another gene (87).
- Recessive epistasis is when the individual must be homozygous for either recessive allele to mask the phenotypic effects of the dominant allele.

2 Gene Interaction and Epistasis

In the case of flowers, homozygosity for the while allele of one gene masks the expression of the purple-producing allele of another gene (88)
Epistasis often occurs because 2 or more different proteins participate in a common function (88).
Complementation is when 2 parents displaying a recessive phenotype produce an offspring with a wild-type phenotype (88).
- This will usually occur because the recessive phenotype in the parents is due to homozygosity at 2 different genes.

Epistasis can produce 3 distinct phenotypes:

Hair color in rats is the example used in the book (88-89).
A gene modifier effect is when the alleles of one gene modify the phenotypic effect of the alleles of a different gene (89).