Line up any class and no two people are the same height, yet every one of them belongs to exactly one blood group. Variation comes in two kinds, continuous and discontinuous, and they have different causes. New alleles, the raw material of all variation, come from mutation.
Variation is the differences between individuals of the same species. Continuous variation gives a range of phenotypes between two extremes (body length, body mass) and is caused by both genes and the environment. Discontinuous variation gives a limited number of phenotypes with no intermediates (ABO blood groups, pea seed shape and colour) and is usually caused by genes only. Mutation is genetic change and is the way new alleles are formed.
Fig. 1: continuous variation gives a histogram with a bell-shaped spread; discontinuous variation gives separate bars with nothing in between
Section 01
Continuous variation.
In the sim a class of 40 people stands on the floor. Press Line up by height and they sort from shortest to tallest. Every height between the shortest and the tallest is possible, and the histogram builds a bell shape: most people near the middle, few at the extremes.
Variation: differences between individuals of the same species. All 40 people are humans, but their heights differ.
Continuous variation: a range of phenotypes between two extremes, with every intermediate value possible. Examples: body length (height) and body mass.
Causes: both genes and the environment. Many genes add small amounts to height, and diet, health and exercise change it too. Drag the Effect of diet (environment) slider up and the spread of heights widens.
Plotting it: group the measurements into classes (for example 155-159 cm, 160-164 cm) and draw a histogram with touching bars.
Examiner trap
Continuous variation is not caused by the environment alone. Set the diet slider to zero in the sim: the heights still vary, because the people have different genes.
Section 02
Discontinuous variation.
Switch between pea seed shape, pea seed colour and ABO blood group, then press Sort them. Every seed or person drops into one of a few clear groups. There is no slightly wrinkled pea and no blood group half way between A and B.
Continuous
Discontinuous
Phenotypes
a range between two extremes
a limited number, no intermediates
Examples
body length, body mass
ABO blood group, pea seed shape (round or wrinkled), pea seed colour (yellow or green)
Caused by
genes and the environment
usually genes only
Graph
histogram, touching bars
bar chart, separate bars
Investigating variation: measure a continuous feature (hand span, height, leaf length) in a large sample, or count individuals in each category of a discontinuous feature (tongue rolling, blood group), then draw the right kind of graph.
Examiner trap
A feature measured in numbers is not automatically continuous. Shoe size is recorded in whole sizes, but foot length is really continuous. Ask whether intermediate values exist in nature.
Section 03
Mutation makes new alleles.
The mutation tab shows one gene as a row of 30 base pairs. Pick a mutagen and press Expose for 10 years. Each base that changes flashes, and the readout counts the bases changed. Compare no mutagen, ionising radiation and a chemical such as tar.
Mutation: a genetic change. It is the way in which new alleles are formed.
Mutagens: ionising radiation (X-rays, gamma rays, radiation from radioactive substances) and some chemicals (such as tar in tobacco smoke) increase the rate of mutation. Mutations still happen without them, but rarely.
Extended.Gene mutation: a random change in the base sequence of DNA. The changed sequence may code for a different protein, giving a new allele.
Extended.Sources of genetic variation in populations: mutation, meiosis, random mating and random fertilisation. Only mutation makes brand new alleles; the other three shuffle existing alleles into new combinations.
Examiner trap
Mutagens do not aim at particular genes. They increase the rate of mutation, but which base changes is random.
Stage 1 · Learn
Check what the sim just showed you
Four quick checks. Each correct answer earns XP and records this skill on your profile.
Quick check+10 XP
In the continuous tab, what happens when the class is lined up by height?
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You set the diet (environment) slider to zero. Why do the heights still vary?
Quick check+10 XP
After sorting pea seeds by shape, how many groups are there?
Quick check+10 XP
In the mutation tab, which choice changes the fewest bases over 10 years?
Stage 2 · Exam
Exam-style questions
Unlocks once the four checks above are done. Worth more XP, written to IGCSE Paper 2 and Paper 4 standard.
Finish the four checks above to unlock the exam questions
Exam style+20 XP
Which feature shows discontinuous variation?
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Identical twins brought up apart differ in body mass but have the same blood group. This shows that:
Exam style+20 XP
Extended: which of these is NOT a source of genetic variation in a population?
Exam style+20 XP
Extended: a gene mutation is best described as:
Skill unlocked
Variation, mastered.
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