The Difference Between Qualitative & Quantitative Traits In Genetics

Our DNA is what codes for all of the genes that are necessary for us to function. These genes also determine our phenotypic traits, which are the traits that make up our observable being. For example, in humans, our hair color and blood type are both traits that are determined by our genetic make-up.

Almost all traits can be divided into two distinct types or categories: qualitative and quantitative traits.

In the general language and vernacular, a "trait" means any sort of quality that someone has, such as a personality trait or the way a person acts. However, the trait definition biology-wise is a bit more specific. In biology, a trait is a characteristic determined by your genetic makeup. Hair color, eye color, size, blood type and hairline are all examples of biological traits in people.

Qualitative Traits in Genetics

Qualitative traits are types of traits that fall into distinct classes or categories without variation within those traits. These types of traits are also referred to as discontinuous traits and discrete traits, since there is no variation outside of the specific, aka discrete, trait classes.

Most qualitative traits in genetics are determined by single genes.

It's often easier to understand this point when looking at examples. A common example can be seen with Mendel's famous pea plant experiments that spawned much of the modern understanding of genetics. Mendel found that these pea plants could produce either plants where the peas were smooth or plants where the peas were wrinkled.

Pea smoothness is a qualitative or discrete trait since there are distinct categories the trait can be. There are no half-wrinkled half-smooth plants or semi-wrinkled plants. They are only smooth or wrinkled, which classifies that trait as qualitative.

Another common example of a qualitative trait in humans is blood type. Humans can have either Rh-positive blood or Rh-negative blood. If you have the gene that codes for the Rh protein in your body, then you'll have a positive blood type (A positive, B positive, O positive, etc). If you lack that gene, then you don't have Rh in your blood and you'll have a negative blood type (A negative, B negative, etc). There's no "in-between" or variation outside of those two discrete options.

These types of traits are also called continuous traits since they usually are displayed on a continuous spectrum, or range, of variation. While qualitative traits are usually determined by single genes, quantitative traits tend to be more complex and are usually controlled by multiple genes. When traits are controlled by more than one gene or by groups of genes, they're called polymorphic traits.

The height of an oak tree would be an example of a quantitative trait in plants. The sawtooth oak tree, for example, ranges in height between 40 and 60 feet. That means these trees could be any height within that range from 40.1 feet to 50.76 feet to 57.01 feet.

The trees don't have height "categories" like in qualitative traits and the trait is seen in variation over a range of options.There also isn't a single "height gene." It's determined by a variety of genes.

Some examples of quantitative traits in humans include:

• Height
• Blood pressure
• Risk of certain diseases
• Intelligence (in terms of IQ)

Each of these traits are influenced by a wide number of genes in complex interactions that determine the physical result. And while you might look at someone and say they're "tall" or "short," those don't indicate specific or discrete categories of the trait as much as they indicate our own perspective on that person's height.