Quantitative inheritance

Continuous and discontinuous variation

Characters can be grouped by the kind of variation they show.

• Qualitative characters (flower colour, seed shape) show discontinuous variation: individuals fall into a few distinct classes. They are usually controlled by one or a few major genes (monogenic or oligogenic), are little affected by environment, and are studied by counting and ratios, as Mendel did.
• Quantitative characters (plant height, grain yield, days to maturity, milk yield, oil content) show continuous variation: values form a smooth range with no clear classes, usually a bell-shaped (normal) curve. They are controlled by many genes (polygenes), each with a small, additive effect, and are strongly influenced by the environment. They are studied by measurement and statistics (mean, variance).

Polygenic inheritance example: Nilsson-Ehle studied wheat kernel colour. With two gene pairs (R₁r₁R₂r₂ × R₁r₁R₂r₂), each dominant allele adds some red colour. The F₂ shows five classes in the ratio 1:4:6:4:1, from dark red to white. With n gene pairs:

• number of phenotypic classes = 2n + 1;
• fraction of F₂ that looks like either extreme parent = (1/4)ⁿ.

For three gene pairs: 7 classes; extreme fraction = 1/64.

Because environment affects quantitative traits, breeders estimate heritability, the proportion of observed variation that is due to genes. High heritability means selection based on appearance works well.

Key points

• Discontinuous: monogenic, distinct classes, little environmental effect.
• Continuous: polygenic, additive, normal distribution, strong environmental effect.
• Two genes (additive): F₂ 1:4:6:4:1.
• Classes = 2n + 1; extreme fraction = (1/4)ⁿ.

Exam tips

Learn to classify examples (yield vs flower colour). For polygene numbers, use the two formulas above and check your answer.

Common mistakes

• Treating yield as a single-gene trait.
• Forgetting that environment changes the expression of quantitative traits.