TRAITS
A trait is any gene-determined characteristic. Many traits are determined by the function of more than one gene. For example, a person’s height is likely to be determined by many genes, including those affecting growth, appetite, muscle mass, and activity level. However, some traits are determined by the function of a single gene.
Variation in some traits, such as eye color or blood type, is considered normal. Other variations, such as albinism, Marfan syndrome, and Huntington disease, harm body structure or function and are considered disorders. However, not all such gene abnormalities are uniformly harmful. For example, one copy of the sickle cell gene can provide protection against malaria, but two copies of the gene cause sickle cell anemia.
In genetics, you can divide all traits into two categories based on their effects on an organism’s phenotype: qualitative and quantitative. Every single genetic trait an organism has fits neatly into one of these two categories. Additionally, if you know that a trait is qualitative or quantitative, you can discern several things about the genes that control these traits.
Qualitative Traits
A qualitative trait is a trait that fits into discrete categories. This means that you can neatly categorize a trait. For example, if a species of plant had either red leaves or yellow leaves, and nothing in between, this would be a discrete trait. “Yes or no” traits, traits where an organism either has the trait or doesn’t, also fit into this category. Usually, a single gene or small group of genes control qualitative traits.
Quantitative Traits
Quantitative traits occur as a continuous range of variation. This means that these traits occur over a range. To picture this, imagine the length of a lizard’s tail. The length can vary, and does not fit into natural categories. Generally, a larger group of genes control qualitative traits. When multiple genes influence a trait, you can also describe it as a “polygenic trait.”
Examples of Qualitative Traits
This concept may make more sense with examples. Some examples of qualitative traits include round/wrinkled skin in pea pods, albinism and humans’ ABO blood groups. The ABO human blood groups illustrate this concept well. Except for some rare special cases, the humans can only fit into one of four categories for the ABO part of their blood type: A, B, AB or O. Since the ABO part of your blood type fits neatly into four categories, it is a qualitative trait. You can often represent qualitative traits with a number.
Examples of Quantitative Traits
Similarly, examples can help people assimilate the idea of quantitative traits. These traits include height, intelligence and skin color. In some organisms, disease resistance is a quantitative trait. Human height illustrates the concept well. Height can occur across a range. While you can say that someone is “short” or “tall,” these are arbitrary values, not innate categories. Instead, the most accurate way to measure height is with a numerical value, making it a quantitative trait.
