Mendelian traits are caused by alleles of a single gene whereas multifactorial or quantitative traits are affected by multiple genes and/or the environment.
The pattern of variation shown by these traits is continuous giving a bell-shaped normal distribution.
Sources of quantitative variation:
- Genotype: alleles of two or more different genes contribute to the phenotype.
- Environment: factors may include nutrition, sunlight, rainfall and density of organisms.
- Genotype-environment interaction: most common cause, seen in humans in the way genotype and nutrition contribute to body height.