The relationship between a gene and a biological effect (genotype and phenotype) can be distinctive. Three relationships can be defined:

(1) single gene-single effect

(2) several genes-single effect or polygenic traits or

(3) single gene-multiple effects or pleiotropic traits.

Review of Mendelian Genetics	Basic Terminology and Concepts Necessary to Understand Genetics
What is our Genetic Makeup - How Does it Work?	Mutations - Source of New Variation
The Bicycle Factory Analogy