Genetics & Inheritance

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.