Describe that discontinuous variation results in a limited number of phenotypes with no intermediates, including ABO blood groups, seed shape and seed colour in peas

Sourav PanNovember 4, 2024

Answered

Discontinuous variation refers to the distinct and separate categories of phenotypes observed within a species, where individuals exhibit clear differences without any intermediates. This type of variation is primarily influenced by genetic factors and is characterized by qualitative traits.

Key Characteristics of Discontinuous Variation

1. Limited Number of Phenotypes: Discontinuous variation results in a finite number of observable traits. For example, in humans, there are four distinct ABO blood groups: A, B, AB, and O. An individual can only belong to one of these categories, with no intermediate blood types existing between them.
2. No Intermediates: Traits associated with discontinuous variation do not have intermediate forms. For instance, in pea plants (Pisum sativum), seed shape can be either round or wrinkled, with no seeds exhibiting a shape that is a blend of both. Similarly, seed color can be either yellow or green, again lacking any intermediate shades.
3. Qualitative Traits: The variations are qualitative rather than quantitative, meaning they are defined by distinct categories rather than a range of measurements. This can be seen in traits like flower color in peas or the presence of specific genetic conditions such as hemophilia, which also exemplifies discontinuous variation.
4. Genetic Basis: Discontinuous traits are typically controlled by single genes or multiple alleles at a single locus. For example, the ABO blood group system is determined by the presence of specific alleles at the ABO gene locus. In peas, the shape and color of seeds are governed by specific genetic mutations that lead to these discrete phenotypes.

Examples

• ABO Blood Groups: The human blood type system illustrates discontinuous variation clearly, as individuals can only have one of four blood types with no intermediates.
• Pea Seed Shape: Gregor Mendel’s studies on pea plants revealed that seed shape is determined by a single gene with two alleles—round (dominant) and wrinkled (recessive)—resulting in either phenotype without any intermediate forms.
• Pea Seed Color: Similar to seed shape, pea seed color can be yellow or green, determined by genetic factors with no shades in between.