QUANTITATIVE TRAITS
Most phenotypic traits in plants and animals are affected by many genes (size, weight, shape, lifespan, physiological traits, fecundity). Often, it is not feasible to determine the number of genes affecting a particular trait, and the individual effects of genes on the phenotype. Many of these traits can be measured on a quantitative, rather than a qualitative, scale. This is where the terms quantitative trait and quantitative genetics come from.
Quantitative traits:
1. Have continuous distributions, not discrete classes
2. Are usually affected by many genes (polygenic)
3. Are also affected by environmental factors
EXAMPLE: COLOR OF WHEAT KERNELS
This trait is determined by two genes that contribute "doses" of red pigment, and display partial dominance (heterozygotes intermediate). Each allele with the subscript "1" contributes 1 dose of red pigment. This trait demonstrates additive effects among different alleles at a single locus, and among alleles at different genetic loci. Genes with subscript "2 " don't contribute any red pigment.
Polygenic inheritance refers to inheritance of a phenotypic characteristic (trait) that is attributable to two or more genes and can be measured quantitatively. Multifactorial inheritance refers to polygenic inheritance that also includes interactions with the environment. Unlike monogenic traits, polygenic traits do not follow patterns of Mendelian inheritance (separated traits). Instead, their phenotypes typically vary along a continuous gradient depicted by a bell curve.
An example of a polygenic trait is human skin color variation. Several genes factor into determining a person's natural skin color, so modifying only one of those genes can change skin color slightly or in some cases, such as for SLC24A5, moderately. Many disorders with genetic components are polygenic, including autism, cancer, diabetes and numerous others. Most phenotypic characteristics are the result of the interaction of multiple genes.
