Genetic vs Environmental Variation — Genetics & Inheritance | IGCSE Biology

Exam Frequency Analysis

Past paper frequency (2018 to 2024)

This topic accounts for approximately 17% of your exam marks.

increasing

Very High

Increasing17%

Genetic crosses, Punnett squares, and dominant/recessive allele questions appear on almost every paper.

Differences between individuals come from genes, environment, or both. It is often useful to ask which kind of variation a particular trait is showing.

Discontinuous variation

A trait that comes in distinct categories, with no in-between values, shows discontinuous variation.

Usually controlled by one or a few genes
Not normally affected by the environment
Examples: blood group (A, B, AB, O), ability to taste PTC (taster vs non-taster), eye colour (broad categories), earlobe attached vs free

Continuous variation

A trait that varies smoothly across a continuous range, with most individuals near an average and fewer at the extremes, shows continuous variation.

Usually polygenic (controlled by many genes acting together) and strongly influenced by the environment
Plot the values on a graph and you get the familiar bell curve
Examples: height (genes + nutrition + childhood illness), body mass (genes + diet + exercise), skin colour (multiple pigment genes + sunlight exposure)

Environmental variation in plants

Plants are especially sensitive to environment. Identical clones (cuttings from the same parent) grown in different conditions can end up quite different:

A shaded plant grows taller and thinner than its sister in full sun
A water-starved plant has smaller leaves
A magnesium-deficient plant has yellow leaves rather than green

So the genotype is the same (they are clones) but the phenotype is different. Environment and genes both matter.