CB3.4 — Variation and mutation (Edexcel 1SC0)

Types of variation

Continuous variation: a range of values with no distinct categories. Examples: height, mass, skin colour. Controlled by many genes + environment.

Discontinuous variation: distinct categories with no intermediates. Examples: blood group, tongue rolling, attached/detached earlobes. Controlled by one or a few genes.

Causes of variation

1. Genetic: differences in alleles (from meiosis, fertilisation, mutations).
2. Environmental: diet, climate, exercise, disease.
3. Both: most human characteristics — e.g. height is genetically determined but also affected by nutrition.

Mutation

A mutation is a random change in the DNA base sequence.

• Mutations occur spontaneously during DNA replication or can be caused by mutagens (UV radiation, X-rays, certain chemicals, some viruses).
• Most mutations are neutral (no effect on phenotype).
• Some are harmful (disrupt protein function — e.g. cause cancer or genetic disease).
• Very rarely mutations are beneficial (improve survival — basis of evolution).

Mutations and evolution

Beneficial mutations increase an organism's fitness → more likely to survive and reproduce → allele becomes more common in the population over generations (natural selection).