Variation

Continuous variation.

• Characteristics that range smoothly between two extremes.
• QUANTITATIVE — measured.
• Examples: HEIGHT, WEIGHT, leaf length, hand span, blood pressure.
• Causes: usually MANY GENES + environment together.
• Plot: SMOOTH CURVE (often bell-shaped — normal distribution).

Discontinuous variation.

• Distinct categories; no intermediates.
• QUALITATIVE — counted.
• Examples: BLOOD GROUP (A/B/AB/O), tongue rolling (yes/no), pea seed colour (yellow/green), eye colour at simple level.
• Causes: usually a SINGLE GENE.
• Plot: BAR CHART with distinct columns.

Examples to memorise:

Trait	Type
Height	Continuous
Weight	Continuous
Hand span	Continuous
Reaction time	Continuous
Blood group	Discontinuous
Tongue rolling	Discontinuous
Pea seed colour	Discontinuous
Earlobe attachment	Discontinuous

Worked qualitative. Why is blood group discontinuous but height continuous?

• Blood group: determined by ONE gene with three alleles (A, B, O). Limited combinations → distinct types.
• Height: result of MANY genes (~700+) plus environment. Each gene a small effect; combined → smooth range.

Cambridge tip. Always pair the type of variation with the type of GRAPH (smooth curve vs bar chart). Cambridge often gives data and asks for the graph.

1. Genetic variation.

Sources:

• Meiosis: random assortment + crossing over → gametes are unique.
• Random fertilisation: any sperm with any egg.
• Mutations: rare random DNA changes; create NEW alleles.

Without these, all individuals would be identical clones (asexual reproduction).

2. Environmental variation.

Same genome, different conditions → different outcomes.

Examples:

• A plant grown in sun vs shade — different leaf size, branching.
• Identical twins raised separately may differ in weight.
• Skin tan from sun exposure.
• Muscle mass from exercise.
• Language and accent — purely environmental.

3. Most traits = both.

• HEIGHT: genes set potential; nutrition determines if you reach it.
• INTELLIGENCE: both genes and education/upbringing matter.
• MANY DISEASES: genetic predisposition + environmental triggers.

Worked qualitative. Identical twins separated at birth often grow up to similar heights but different weights. Why?

• Identical twins = same genes.
• Height ~80% genetic, weight ~50% genetic.
• Different diets and lifestyles affect weight more than height.
• An example of how environment affects different traits to different degrees.

Cambridge tip. When asked about CAUSE of variation, give BOTH genetic and environmental sources. Just one is partial.

Definition. A mutation is a random change in the DNA sequence (or chromosome structure).

Where mutations come from:

• Random copy errors during DNA replication (most common).
• Ionising radiation: X-rays, UV light, gamma rays.
• Chemicals (mutagens): tobacco smoke (carcinogens), pesticides, some medications.

Types of mutation:

• Point mutation: a single base changes.
• Insertion / deletion: bases added or removed.
• Chromosomal: large segments rearranged.

Effects:

• Most mutations are NEUTRAL (in 'silent' DNA or have no protein-changing effect).
• Some are HARMFUL (cause disease; cancer; birth defects).
• A small fraction are BENEFICIAL (increase survival/reproduction in some environment).

Why mutations matter:

• Without mutations, no new alleles arise.
• Without new alleles, evolution can't happen.
• Mutations introduce variation that natural selection acts on.

Examples of beneficial mutations:

• Lactase persistence in adult humans (allows milk digestion). Spread in herding cultures.
• Sickle-cell allele: harmful to homozygotes but heterozygotes resistant to malaria.
• Bacterial resistance to antibiotics.

Worked qualitative. Why is exposure to UV radiation linked to skin cancer?

• UV light causes DNA mutations in skin cells.
• Some mutations affect genes controlling cell division.
• Uncontrolled cell division → tumours → cancer.
• Reasons to use sunscreen and avoid sunbeds.

Cambridge tip. Always say 'random change in DNA' — the word RANDOM matters. Mutations are not directed by environment; the environment just SELECTS which random mutations spread.