14.1 Variation

A free GCSE Biology lesson for AQA (8461) from OBA School: genetic and environmental causes of variation, continuous and discontinuous data, and mutations. Watch the video, read the explanation and worked examples, then try the revision questions. You can work through the full lesson, with every video, practice quizzes and printable worksheets with mark schemes, in our free online course.

What you will learn

• Explain that variation can be caused by genes, by the environment, or by both
• Use twin studies and other data to decide which cause matters more
• Tell continuous and discontinuous variation apart and draw the right graph for each
• Describe how mutations produce new genetic variants

Key words

• Variation: Differences in the characteristics of individuals in a population.
• Genetic variation: Differences caused by the genes (alleles) an individual inherits from its parents.
• Environ- mental variation: Differences caused by the conditions in which an organism develops, such as diet, light or lifestyle.
• Phenotype: The characteristics an organism actually shows.
• Continuous variation: A characteristic that can have any value in a range, e.g. height or leaf length.
• Discontinuous variation: A characteristic that falls into separate groups, e.g. blood group.
• Mutation: A change in the DNA. Mutations happen all the time and create new genetic variants.

Genes, environment, or both?

No two people are exactly alike. The differences between individuals of the same species are called variation. They come from three sources:

• Genes only; Examples in humans Blood group, inherited disorders such as cystic fibrosis; Examples in plants Flower colour in many species
• Environment only; Examples in humans Scars, tattoos, the language you speak; Examples in plants Leaves lost to a grazing animal
• Both; Examples in humans Height, body mass, skin colour; Examples in plants Height and number of fruits (depend on light, water and minerals too)

Key fact: Most characteristics are a mix: genes set the possible range, and the environment decides where in that range an individual ends up.

Using twins to separate the causes

Identical twins come from one fertilised egg, so they have the same genes. Any difference between them must be caused by the environment. Non-identical twins share only about half their genes, like ordinary brothers and sisters.

• Identical, raised together; Mean difference in height (cm) 1.6; Mean difference in mass (kg) 1.9
• Identical, raised apart; Mean difference in height (cm) 1.8; Mean difference in mass (kg) 4.6
• Non-identical, raised together; Mean difference in height (cm) 4.3; Mean difference in mass (kg) 4.8

Worked example: interpreting twin data

Height: identical twins raised apart differ by only 1.8 cm, almost the same as those raised together (1.6 cm). A different environment made little difference, so height is mainly controlled by genes.
Mass: identical twins raised apart differ by 4.6 kg, much more than those raised together (1.9 kg). Changing the environment had a big effect, so mass is strongly affected by the environment (diet and exercise).

Answer: height mainly genetic; mass strongly affected by the environment

Continuous and discontinuous variation

Continuous data can take any value in a range. It is plotted as a histogram (bars touching) and often makes a bell-shaped "normal distribution". Discontinuous data falls into categories with nothing in between, so it is plotted as a bar chart with gaps.

Worked example: working with the data

What percentage of the 50 people in Figure 1 have blood group O? 22 ÷ 50 × 100 = 44%
How many students have a hand span of 20 cm or more? 17 + 11 + 5 + 2 = 35

Answer: 44% are group O; 35 students have a span of at least 20 cm

Mutations: the source of new variants

There is usually a lot of genetic variation within a population. All genetic variants first arise from mutations – changes in DNA. Mutations happen continuously.

• Most mutations have no effect on the phenotype.
• Some influence the phenotype a little.
• Very few decide the phenotype on their own.

Very rarely, a mutation produces a new phenotype. If that phenotype happens to suit a change in the environment, it can lead to a relatively rapid change in the species (see 14.2).

Key fact: Sexual reproduction shuffles existing alleles into new combinations, but only mutation makes completely new variants.

Common mistakes

• Saying identical twins are identical in every way. They share genes, but their environments can make them differ.
• Putting every characteristic into "genetic" or "environmental". Many, like height and mass, depend on both.
• Drawing a bar chart with gaps for continuous data. Use a histogram with touching bars.
• Saying all mutations are harmful. Most have no effect on the phenotype at all.

Revision questions

Try these, then check your answers below each one.

Question 1. Which of these shows continuous variation? (Choose from: Blood group, Whether a pea seed is round or wrinkled, Biological sex (male or female), Hand span.)

Answer: Hand span

Working: Hand span can take any value in a range. The others fall into separate categories (discontinuous).

Question 2. True or false? Identical twins who are raised apart can have different body masses.

Answer: True

Working: True. They share genes, but different diets and lifestyles (environment) can change body mass.

Question 3. Which statement about mutations is correct? (Choose from: Every mutation produces a new phenotype, Most mutations have no effect on the phenotype, All mutations are harmful, Mutations happen only when an organism needs to change.)

Answer: Most mutations have no effect on the phenotype

Working: Most mutations have no effect; some influence the phenotype; very few decide it. They are random, not caused by need.

Related lessons

• Previous lesson: 13.10 Screening for Genetic Disorders
• Next lesson: 14.2 Evolution by Natural Selection
• In this chapter: 14.3 Selective Breeding
• In this chapter: 14.4 Genetic Engineering
• In this chapter: 14.5 Cloning

Continue in the full course

This lesson is section 14.1 (AGCBH1401) of our free GCSE Biology course for AQA. In the full course you get every video for this section, a self-marking practice quiz, a printable worksheet with a mark scheme, chapter tests, and your progress saved. Go to the full GCSE Biology course.