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B13.1 Variation

GCSE Combined Science · AQA · Chapter B13: Variation and Evolution

A free GCSE Combined Science: Trilogy lesson for AQA (8464) from OBA School: genetic and environmental causes of variation, continuous and discontinuous data, normal distribution 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
  • Tell continuous and discontinuous variation apart and choose the right graph for each
  • Describe a normal distribution and explain why large samples are better
  • Describe how mutations produce new genetic variants and how often they affect the phenotype

Key words

  • Variation: Differences in the characteristics of individuals in a population.
  • Genetic variation: Differences caused by the alleles an individual inherits from its parents.
  • Environ- mental variation: Differences caused by the conditions in which an organism develops, such as diet, light, sunlight or injury.
  • Continuous variation: A characteristic that can have any value in a range, e.g. height or foot length.
  • Discontinuous variation: A characteristic that falls into separate groups with nothing in between, e.g. blood group.
  • Normal distribution: A bell-shaped spread of data: most values near the middle, few at each end.
  • Mutation: A change in DNA. Mutations happen all the time and are the source of new genetic variants.

Genes, environment, or both?

The differences in the characteristics of individuals in a population are called variation. Variation in the phenotype may be due to differences in:

  • Genes only; Examples in humans blood group, natural eye colour, cystic fibrosis; Examples in plants flower colour in many species
  • Environment only; Examples in humans scars, tattoos, hair lightened by the Sun, the language you speak; Examples in plants leaves eaten by a caterpillar
  • 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: For most characteristics, genes set a possible range and the environment decides where in that range an individual ends up. Example: children of tall parents tend to be tall, but a poor diet can stop them reaching that height.

Worked example: genes or environment?

Two cuttings are taken from one geranium, so they are clones. One grows on a sunny windowsill and reaches 42 cm; the other grows in a dark corner and reaches 25 cm. What causes the difference?
They have identical genes, so the difference must be environmental: more light means more photosynthesis and more growth.

Answer: the environment (light)

Continuous and discontinuous variation

Continuous data can take any value in a range (152 cm, 152.5 cm…). It is shown as a histogram (bars touching) or a line graph. Discontinuous data falls into separate groups, so it is shown as a bar chart with gaps.

Worked example: working with the data

What percentage of the 50 people have brown eyes? 26 ÷ 50 × 100 = 52%
How many students have a foot length of 24 cm or more? 19 + 12 + 6 + 2 = 39

Answer: 52% have brown eyes; 39 students

Normal distribution

Many continuous characteristics are normally distributed: most individuals have values near the middle and few are at each extreme, giving a bell-shaped graph. The peak of the curve is the most common value.

Exam tip: A small sample (one class) may not give a smooth bell shape. A larger sample (a whole year group) gives a better shape and results that are more likely to be repeatable.

Mutations: the source of new variants

There is usually extensive genetic variation within a population of a species. All new genetic variants first arise from mutations: changes in DNA. Mutations happen continuously.

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

Very rarely a mutation leads to a new phenotype. If the new phenotype is suited to an environmental change, it can lead to a relatively rapid change in the species (see B13.2).

Key fact: Sexual reproduction makes new combinations of existing alleles; only mutation makes brand-new alleles.

Common mistakes

  • 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 bars touching.
  • Saying all mutations are harmful. Most have no effect on the phenotype at all.
  • Saying identical twins are identical in every way. They share genes, but the environment can make them differ.

Revision questions

Try these, then check your answers below each one.

Question 1. The histogram shows the heights of 80 sunflowers. Which best describes the data? (Choose from: Continuous variation with a normal distribution, Discontinuous variation with a normal distribution, Continuous variation with most plants at the extremes, Discontinuous variation in four groups.)

Answer: Continuous variation with a normal distribution

Working: Height can take any value in a range, so it is continuous. Most plants are near the middle (160–180 cm) and few are at the ends, so it is a bell-shaped normal distribution.

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

Answer: True

Working: Identical twins have the same genes, but body mass is also affected by the environment (diet and exercise), so twins raised apart can differ.

Question 3. Which statement about mutations is correct? (Choose from: Most mutations have no effect on the phenotype, All mutations are harmful, Mutations only happen when the environment changes, Most mutations decide the phenotype on their own.)

Answer: Most mutations have no effect on the phenotype

Working: Mutations happen continuously. Most have no effect on the phenotype, some influence it and very few determine it.

Related lessons

Continue in the full course

This lesson is section B13.1 (AGCSC1301) of our free GCSE Combined Science 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 Combined Science course.

 

 

 

 

 

 

 

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