17.1.2—Discontinuous vs continuous variation

Discontinuous variation places individuals into distinct, recognisable categories. Continuous variation gives a measurable range, with intermediate values between the extremes.

• Discontinuous: the phenotype is qualitative and category-based; a value does not fall between the categories. Example: ABO blood groups.
• Continuous: the phenotype is quantitative and measured on a scale; individuals can take values across a range. Examples: human height or mass.
• In a table or graph, separated categories support a discontinuous interpretation, whereas a spread of values across a range supports a continuous interpretation.

The pattern describes what is observed, not automatically why it occurs. Discontinuous traits are often associated with large effects at one gene locus, while continuous traits commonly reflect many small genetic effects together with environmental influence; the genetic basis must be checked rather than inferred from the appearance alone.

Continuous does not mean “not genetic”, and discontinuous does not by itself prove a single-gene cause. This card classifies the observed variation; it does not develop the detailed genetic architecture or apply a t-test to sample means.