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Biology · Inheritance

Variation and Mutations

No two people are exactly alike, and neither are two peas in a pod. This lesson covers continuous and discontinuous variation, the causes of variation, and how mutations produce the variants that are the source of all of it.

  • 9 key terms
  • All boards

Learning Objectives

  1. 1Describe, with examples, discontinuous variation such as eye colour and continuous variation such as height and weight.
  2. 2Describe the genome and the environment as causes of variation in the phenotype.
  3. 3Recall that all variants arise from mutations, and that most have no effect on the phenotype, some influence the phenotype and a very few determine the phenotype.
  4. 4Process and interpret data on variation, including histograms and bar charts.

Retrieval practice

  1. 1

    What is a mutation?

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    A change in the DNA base sequence.

  2. 2

    What is a variant?

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    A different version of a gene, also called an allele.

  3. 3

    What does the genome interact with to influence the phenotype?

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    The environment.

  4. 4

    What is the mean?

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    The sum of the values divided by the number of values.

Why individuals differ

Look at the members of your class. They are all human beings, but they differ in height, mass, eye colour, blood group and many other features. These differences between individuals of the same species are called variation. Variation may be caused by the genes that a person has inherited, by the environment in which they have lived, or, most often, by both. Whatever the cause, all of the variants that exist today arose from mutations.

Discontinuous variation

In discontinuous variation, the feature falls into distinct categories, with no values in between.

  • Examples

    Eye colour is an example of discontinuous variation, as are blood group, whether you can roll your tongue, and whether a pea is round or wrinkled.

  • Controlled by

    These features are often controlled by one gene, or a small number of genes, and are little affected by the environment.

  • Shown by

    The data are shown by a bar chart, with gaps between the bars, or a pie chart.

Continuous variation

In continuous variation, a feature can have any value within a range.

  • Examples

    Height and weight are examples of continuous variation, as are the length of a leaf and a person's heart rate.

  • Controlled by

    These features are the result of many genes, together with the effect of the environment.

  • Shown by

    The data are shown by a histogram, with touching bars, or a line graph. A large sample often gives a bell-shaped curve.

Continuous and discontinuous variation

Continuous variation

  • Any value in a range, with no clear groups
  • Examples: height and weight
  • Many genes, and often the environment as well
  • Drawn as a histogram or a line graph

Discontinuous variation

  • Distinct categories, with no in-between values
  • Examples: eye colour and blood group
  • One or a few genes, with little effect of the environment
  • Drawn as a bar chart

Genes, environment, or both

The cause of the variation in a feature can be described in three ways.

  • Genetic causes

    Eye colour, blood group and natural hair colour are caused by the genes that a person inherits.

  • Environmental causes

    Scars, a suntan, a person's accent and the language that they speak are caused by the environment and are not inherited.

  • Both

    Many features, such as height, body mass and skin colour, are influenced by the genome and by the environment together.

  • Why it matters

    The genome sets a range, and the environment decides where in the range an individual ends up. Only the genetic variation can be passed on.

Variants come from mutations

Every variant started as a mutation, a random change in the base sequence of DNA.

  • All variants arise from mutations

    A new variant, or allele, can only be made by a mutation. Sexual reproduction mixes existing variants into new combinations, but it cannot make new ones.

  • Most have no effect

    Most mutations have no effect on the phenotype. They may be in DNA that does not code for a protein, or may not change the protein that is made.

  • Some influence the phenotype

    A smaller number of mutations change the protein or the amount of it, and so have a small effect on the phenotype.

  • A very few determine the phenotype

    A very few mutations have a large effect, and decide the phenotype, such as the one that causes cystic fibrosis.

  • What causes mutations

    They occur at random, and are more likely if cells are exposed to ionising radiation, ultraviolet light or some chemicals, which are called mutagens.

Mean and mode of a sample

A student measured the length of ten leaves from the same tree: 5, 6, 6, 7, 7, 7, 8, 8, 9 and 9 cm. Calculate the mean, state the mode, and say whether this is continuous or discontinuous variation.

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  1. 1 Add the lengths \(5 + 6 + 6 + 7 + 7 + 7 + 8 + 8 + 9 + 9 = 72\) cm.
  2. 2 Find the mean \(72 \div 10 = 7.2\) cm.
  3. 3 Find the mode The most common length is 7 cm.
  4. 4 Classify Leaf length can have any value in a range, so it is continuous variation.

AnswerThe mean is 7.2 cm, and the mode is 7 cm. The variation is continuous.

Investigating variation

A common investigation looks at one feature in a sample of one species.

  • Choose a feature

    For example the length of leaves, or the arm span of students, which is continuous, or whether students can roll their tongues, which is discontinuous.

  • Sample

    Use a random sample, of at least 30 individuals, so that the sample is not biased and the results are reliable.

  • Measure

    Measure each in the same way, and record the results in a table.

  • Process

    Group continuous data into classes, draw a histogram, and calculate the mean. Draw a bar chart for discontinuous data.

  • Conclude

    Say which type of variation is shown, and what the likely causes are.

Case study

Identical twins

Identical twins have exactly the same genome, so any differences between them must be caused by their environment. Studies of twins who grew up in different families show which features are strongly controlled by genes, and which by the environment. Eye colour and blood group are almost always the same, but height and body mass can differ if the diet is different. They show that variation in many features is caused by a mixture of genes and the environment.

Same genome Identical twins are genetic clones.
Differences Must be caused by the environment.

Summary and exam focus

  • Discontinuous variation falls into distinct categories, such as eye colour.
  • Continuous variation can take any value in a range, such as height and weight.
  • Variation is caused by the genome, by the environment, or by both.
  • All variants arise from mutations.
  • Most mutations have no effect, some influence the phenotype, and a very few determine it.

Exam focus

Describe the difference between continuous and discontinuous variation, giving an example of each, and explain how mutations give rise to variation. (6 marks) (6 marks)

Define each type of variation, and give the OCR examples: eye colour for discontinuous, and height or weight for continuous. Then say that all variants arise from mutations, and that most have no effect, some influence the phenotype and a very few determine it.

Key terms

The words this lesson expects you to use. Each one is linked from the first place it appears above.

Variation
The differences between individuals of the same species.
Discontinuous variation
Variation in which individuals fall into distinct categories.
Continuous variation
Variation in which a feature can have any value within a range.
Mutation
A random change in the base sequence of DNA.
Variant
A different version of a gene, which arose from a mutation.
Mutagen
Something that increases the rate of mutation, such as ionising radiation.
Histogram
A bar chart for continuous data, in which the bars touch.
Mode
The value that occurs most often.
Environment
The surroundings and conditions in which an organism lives.

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