Some Key Terms

In Figure 2, how many differences do you see between the two plants on the left compared to the two plants on the right?

Figure 2:  The two plants on the left belong to the variety Longfellow, whereas the two plants on the right belong to the variety Gourdseed. All four plants were grown in the same field at Lincoln, Nebraska. (UNL, 2004)

Clearly, they differ in height. They also differ in the shape and size of the tassel. Plant height and tassel shape and size are examples of phenotype. Even bands revealed by a molecular digestion of genetic material (DNA) from a plant (Figure 3) is an example of a phenotype. A phenotype is any observable characteristic.

Figure 3:  Stained DNA observed observed moving through a gel reveals a molecular phenotype. (UNL, 2004)

For some traits, phenotypes are easily classified (example: the round, smooth vs. wrinkled seeds of pea plants that were observed and reported by Mendel in the 1860’s). Such traits are referred to as qualitative traits. But for quantitative traits, phenotypes do not fall into such distinct classes. Therefore, phenotypes of quantitative traits typically are measured by some tool, such as a ruler or scale, and the resulting value is known as a phenotypic value.

Although qualitative and quantitative traits differ in how their phenotypes are assessed, both are inherited according to Mendel’s laws of inheritance (refer to the lesson, Just the facts, for a review of these laws). Usually only one or a few genes control the expression and inheritance of a qualitative trait, whereas a quantitative trait is often controlled by more than a few genes.

Why do the two pairs of plants in Figure 2 differ in plant height and other characteristics? There may be two reasons:

  • They may have different genes for these traits.  A gene is a unit of molecular information that controls cellular development and/or activity and is inherited from one’s parent(s).  The collection of genes possessed by an individual is known as its genotype.
  • A second factor that influences phenotype is the environment. For example, farmers are well aware that any single-cross hybrid of corn (a single-cross hybrid is a type of variety) will produce more grain yield when rainfall is plentiful than when a drought occurs, even though all plants of a single cross are genetically identical.