explain the genetic basis of discontinuous variation and continuous variation

Variation in A‑Level Biology

Discontinuous Variation

Discontinuous variation is like picking a fruit from a basket: you either get a red apple or a green apple, with no shades in between. In genetics, this means a trait is controlled by a single gene (monogenic) and shows only a few distinct phenotypes. Examples: pea plant flower colour, blood type, presence/absence of a trait such as a widow’s peak.

• Pea flower colour: $A$ (purple) vs. $a$ (white)
• Blood type: $IA$, $IB$, $i$ (ABO system)
• Widow’s peak: $W$ (present) vs. $w$ (absent)

Continuous Variation

Continuous variation is like measuring the height of students: you get a smooth range of values, not just a few distinct options. This occurs when many genes (polygenic) and environmental factors influence the trait. Examples: height, skin colour, eye colour, weight.

• Height: $AA$, $Aa$, $aa$ (and many more loci)
• Skin colour: a gradient from light to dark
• Eye colour: blue, green, brown, with intermediate shades

Key Differences in a Table

Exam Strategy Checklist

1. Identify if the trait shows discontinuous or continuous variation.
2. Explain the genetic basis: monogenic vs. polygenic.
3. Use appropriate terminology: dominant, recessive, co‑dominant, additive, environmental.
4. Provide a clear example and, if possible, a simple diagram or equation (e.g., $P = \frac{1}{4}$ for a recessive trait).
5. Discuss how environmental factors can modify the phenotype in continuous variation.

Quick Recap