The difference between continuous and discontinuous variation, the genetic and environmental causes of variation, how variation data is presented, and the role of mutation in producing new variation.

What this dot point is asking

CCEA wants you to distinguish continuous and discontinuous variation, describe the genetic and environmental causes of variation, explain how variation data is presented, and explain the role of mutation in producing new variation.

Continuous and discontinuous variation

Causes of variation

Presenting variation data

Continuous data (a range) is shown on a histogram or line graph, which often forms a bell-shaped curve. Discontinuous data (categories) is shown on a bar chart, with a separate bar for each category.

The role of mutation

Examples in context

Example 1. Why identical twins can still differ

Identical twins have the same genes, so any differences between them must be environmental. If one twin eats more and exercises less, they may be heavier; if one spends more time in the sun, their skin may be darker. Comparing identical twins is a classic way to separate genetic from environmental causes of variation, and it shows that a feature such as body mass is influenced by both genes and the environment.

Example 2. How mutation feeds variation

A mutation in a bacterium might change a gene so that the bacterium can survive an antibiotic. This new allele did not exist before; it arose by a random change in the DNA. Because the mutation gives an advantage, the bacterium survives and passes the allele on. This example shows mutation as the ultimate source of new variation, and it links variation directly to natural selection and antibiotic resistance.

Example 3. Why most human features are continuous

Many human characteristics, such as height, mass and skin colour, are controlled by several genes working together as well as by the environment, so they vary smoothly across a range rather than falling into a few categories. Height, for example, depends on many inherited alleles and on diet during childhood. Because lots of small factors add up, the values spread out into a continuous range that forms a bell-shaped curve on a histogram. In contrast, features controlled by a single gene with a few alleles, such as blood group or tongue rolling, give the distinct categories of discontinuous variation. Recognising that the number of genes and the influence of the environment decide which type of variation you see is exactly the reasoning CCEA wants you to apply.

Try this

Q1. Give one example of discontinuous variation in humans. [1 mark]

• Cue. Blood group (or tongue rolling, or ear lobe type).

Q2. What is a mutation? [1 mark]

• Cue. A random change in the base sequence of DNA, creating a new allele.