What Is a Quantitative Trait?

In genetics, a quantitative trait is one that varies in its characteristics and expressions. Two or more genes, along with environmental interactions, are usually responsible for the ultimate expression. Such traits are described in numbers and may vary in degrees. Most quantitative traits are continuous and often do not fall into any discrete categories. The study of the inheritance of such traits is called “Quantitative Genetics,” and such phenotypes in humans include — but are not limited to — height, intelligence quotient (IQ) and blood pressure.

When scientists know a genotype or the internal genetic code for a specific trait, they can predict the resulting phenotypes or outward characteristics of that trait. These are referred to as discontinuous traits and are assigned discrete classes. Not all traits, however, fall neatly into a discrete classification, but instead are continuous and hard to predict. Such types of traits are called quantitative traits, because they are usually recorded as number distributions.

Polygenic inheritance is the term used by scientists to describe the formation of a quantitative trait. Genes that influence the value of quantitative traits are referred to as quantitative trait locus (QTL). The formation of quantitative traits involves two or more genes contributing to a phenotypic characteristic and, often, also involves interaction with the environment. Height in humans, for example, involves a number of genes; however, the ultimate expression of the gene is impacted by environmental factors, such as available nutrition. Rather than following a specific pattern, the traits vary along a continuous gradient that is often illustrated on a bell curve.

Numbers and percentages are the primary methods of documenting quantitative traits. Numerical values of quantitative traits are often ordered from the highest to lowest and depict a continuous order rather that a specified count along a continuous gradient. Values of quantitative traits often differ only in small, arbitrary amounts rather than by fixed amounts. Therefore, most scientists assume continuous data rather than specific values or counts when documenting quantitative traits. Scientists attempt to predict the variance of a quantitative trait to find the mean of distribution and depict the spread on a bell curve.

Expressions of quantitative traits appear in just about every organism, often notably in plant and animal life. In plants, examples include crop yield, color distribution and disease resistance. With animals and people weight, height, learning ability and even blood pressure are each expressed as a quantitative trait.