explain what is meant by discontinuous variation and continuous variation

Published by Patrick Mutisya · 14 days ago

Cambridge A‑Level Biology 9700 – Variation

Variation

Variation refers to the differences that exist between individuals of the same species. These differences can be observed in morphology, physiology, behaviour or biochemical traits. In the A‑Level syllabus variation is divided into two main categories:

  1. Discontinuous (or qualitative) variation

  2. Continuous (or quantitative) variation

Discontinuous \cdot ariation

Discontinuous variation is characterised by distinct, separate forms that can be clearly distinguished from one another. The traits are usually controlled by one or a few genes and the phenotypes fall into discrete categories.

  • Often described as “all‑or‑nothing” traits.

  • Typical examples include blood groups (A, B, AB, O), presence or absence of a tail in certain insects, or flower colour in peas (purple vs. white).

  • Inheritance follows Mendelian patterns (dominant, recessive, co‑dominant, sex‑linked).

  • Population frequencies can be expressed as percentages of individuals in each category.

Continuous \cdot ariation

Continuous variation shows a range of phenotypes that blend into one another without clear boundaries. These traits are usually polygenic (controlled by many genes) and are also influenced by the environment.

  • Traits are measured on a scale (e.g., height, seed weight, enzyme activity).

  • The distribution of values in a large population often approximates a normal (bell‑shaped) curve.

  • Environmental factors such as nutrition, temperature or light can shift the mean of the distribution.

  • Mathematically, the phenotype \$P\$ can be expressed as \$P = G + E\$, where \$G\$ is the genetic contribution and \$E\$ the environmental contribution.

Comparison of Discontinuous and Continuous \cdot ariation

FeatureDiscontinuous \cdot ariationContinuous \cdot ariation
Genetic basisOne or few genes (Mendelian)Many genes (polygenic) + environment
Phenotypic categoriesDistinct, separate classesRange of values forming a continuum
Typical examplesBlood type, flower colour in peas, presence of a traitHuman height, seed mass, enzyme activity
Distribution in populationBar chart of frequencies for each classNormal (bell‑shaped) curve
Effect of environmentLittle or noneSignificant; can shift mean or variance

Why the Distinction Matters

Understanding the type of variation is essential for predicting inheritance patterns, designing breeding programmes, and interpreting evolutionary processes. Discontinuous traits are useful for studying simple genetic mechanisms, whereas continuous traits illustrate the combined effects of genetics and environment, highlighting concepts such as heritability and natural selection.

Suggested diagram: A side‑by‑side illustration showing (a) a bar chart of blood‑type frequencies (discontinuous) and (b) a normal distribution curve for human height (continuous).