IGCSE · Practice questions · Variation

Range or category?

Six original Cambridge-style questions on the two types of variation, drawing the right graph, genes versus environment, mutagens and where genetic variation comes from.

Original questions All questions on this page are original work, written in the Cambridge IGCSE style. They are not from past papers. They test the same concepts and skills the syllabus rewards.

Keep these straight

Two kinds, two causes.

Continuous: range between two extremes; genes and environment; histogram.
Discontinuous: a few distinct phenotypes, no intermediates; usually genes only; bar chart.
Mutation: genetic change; makes new alleles; rate raised by ionising radiation and some chemicals.
Extended: sources of genetic variation are mutation, meiosis, random mating and random fertilisation.

Recall

[3 marks]

Define variation and state one example each of continuous and discontinuous variation in humans.

Variation: differences between individuals of the same species. ✓
Continuous: body length (height) or body mass. ✓
Discontinuous: ABO blood group. ✓

Data

[4 marks]

A student measured the hand span of 30 classmates.

Hand span / cm	16-17	18-19	20-21	22-23	24-25
Number of students	2	7	12	6	3

(a) State the type of variation shown and the type of graph you would draw. (b) Describe the pattern. (c) Suggest why hand span shows this type of variation.

(a) Continuous variation; a histogram (bars touching). ✓
(b) Most students are in the middle class (20-21 cm) and fewer towards each extreme. ✓
(c) Hand span is affected by many genes and by the environment (for example diet during growth), ✓
so every value between the extremes is possible. ✓

Explain

[3 marks]

Pea plants produce seeds that are either round or wrinkled. Explain why this is described as discontinuous variation and state what usually causes it.

There is a limited number of phenotypes (two). ✓
There are no intermediates: no seeds are partly wrinkled. ✓
It is usually caused by genes only, not by the environment. ✓

Recall

[3 marks]

Describe what is meant by mutation and name two factors that increase the rate of mutation.

Mutation is a genetic change; it is how new alleles are formed. ✓
Ionising radiation, for example X-rays or gamma rays. ✓
Some chemicals, for example tar in tobacco smoke. ✓

Analysis

[3 marks]

A gardener grows plants from cuttings of one parent plant, so they are genetically identical. Some grow in shade and some in full sun. The plants in the sun grow taller. Explain what this shows about the cause of variation in height, and why the plants all have the same flower colour.

The plants have the same genes, so the difference in height must be caused by the environment (light). ✓
Height shows continuous variation, which is affected by the environment as well as genes. ✓
Flower colour is (usually) discontinuous and controlled by genes only, and the genes are identical, so the colour is the same. ✓

Explain

[4 marks]

Extended. Describe a gene mutation and name the other sources of genetic variation in a population. State which source is the only one that produces new alleles.

A gene mutation is a random change in the base sequence of DNA. ✓
Other sources: meiosis (it produces gametes that are genetically different from each other), ✓
random mating and random fertilisation (any gamete can fuse with any other). ✓
Only mutation produces new alleles; the others create new combinations of existing alleles. ✓

Mark this once you have attempted all six and checked your working. It records a Practiced badge on the topic and adds a one-time bonus. Revealing the solutions alone does not count.