Living organisms display a wide range of observable characteristics, and these differences among individuals within a species are known as variation. Variation can be classified in various ways, providing insight into its underlying causes. One distinct category of such differences is discontinuous variation, which describes traits with clear-cut, separate forms.
Defining Discontinuous Variation
Discontinuous variation refers to traits that fall into distinct, separate categories with no intermediate forms between them. This type of variation is described as qualitative, meaning the differences are categorical rather than measurable along a spectrum. For instance, a characteristic is either present or absent, or it falls into one of a few defined types.
These traits are controlled by a single gene or a small number of genes, with each gene having a large effect on the observable trait. The genetic basis for discontinuous variation involves Mendelian inheritance patterns, where distinct genetic factors determine phenotypic outcomes. Environmental factors have minimal or no direct influence on the expression of discontinuous traits, meaning the phenotype is primarily determined by the genotype. When represented graphically, data for discontinuous variation appears as distinct bars, reflecting the clear categories rather than a continuous curve.
Examples of Discontinuous Variation
Several traits in both humans and other organisms illustrate discontinuous variation. A well-known example in humans is the ABO blood group system, where individuals possess one of four distinct blood types: A, B, AB, or O. There are no intermediate blood types, and a person unequivocally belongs to one of these categories.
Another common human example is the ability to roll one’s tongue into a U-shape. This trait is controlled by a single gene. In plants, flower color can also exhibit discontinuous variation, such as pea plants having either purple or white flowers. Similarly, seed shape in pea plants, being either round or wrinkled, is another example influenced by a single gene.
Discontinuous vs. Continuous Variation
Understanding discontinuous variation is enhanced by contrasting it with continuous variation. Continuous variation refers to characteristics that show a wide range of values, with gradual transitions between extremes. These traits do not fit into distinct categories but rather exist on a spectrum, such as human height, weight, or skin color. Unlike the clear-cut categories of discontinuous traits, continuous traits can take on any value within a given range.
The genetic control of continuous variation is more complex, involving multiple genes, referred to as polygenic inheritance, with each contributing a small additive effect to the overall trait. Environmental factors also play a significant role in shaping continuous traits, influencing their expression alongside genetic predispositions. For example, a person’s height is influenced by both genetics and nutrition. In contrast, discontinuous variation is primarily determined by genetics alone, with minimal environmental influence. When plotted on a graph, continuous variation forms a smooth, bell-shaped curve, indicating a normal distribution of values, whereas discontinuous variation shows separate, distinct peaks.