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Biology · Reproduction, Meiosis and Variation

Mutations and Genetic Variation

Every new allele began as a mutation. This lesson explains what mutations are, what causes them, why most of them do nothing, and why a population of a species usually has a lot of genetic variation.

  • 9 key terms
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Learning Objectives

  1. 1Describe a mutation as a random change in the DNA of an organism.
  2. 2State the factors that increase the rate of mutation.
  3. 3Explain that most mutations have no effect on the phenotype, some have a small effect and a few have a large effect.
  4. 4Explain why there is usually extensive genetic variation in a population, and why this matters.

Retrieval practice

  1. 1

    What is an allele?

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    A different version of the same gene.

  2. 2

    What is genetic variation?

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    Differences in characteristics caused by differences in genes.

  3. 3

    What is a gene?

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    A section of DNA that codes for a protein.

  4. 4

    What do we call the full set of DNA in an organism?

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    The genome.

Where new alleles come from

Meiosis and fertilisation mix up existing alleles, but they cannot make new ones. New alleles come from mutations. A mutation is a change in the sequence of bases in DNA. It happens at random, and it can happen in any cell, in any gene or between genes. Over many generations, mutations build up in a population, so there is usually a lot of genetic variation in a species. This variation is the raw material on which natural selection can act.

What is a mutation

A mutation is a random change to the base sequence of DNA.

  • A change in the bases

    One or more bases are swapped, added or deleted. This changes the sequence along the DNA molecule.

  • Random

    Mutations happen by chance. They can occur anywhere in the genome, and nobody can predict where or when.

  • Spontaneous

    Most happen when DNA is copied before cell division, and the copying process makes a mistake.

  • New allele

    A mutation in a gene can produce a new allele of that gene, which may give the protein a different structure.

What increases the rate of mutation

Mutations happen naturally at a low rate, but certain things make them more likely. These are called mutagens.

  • Ionising radiation

    X-rays, gamma rays and ultraviolet light can damage DNA. Radiation is a risk to people who work with it, such as radiographers.

  • Chemicals

    Some chemicals, such as those in tobacco smoke and some industrial substances, can damage DNA.

  • Which cells matter

    A mutation in a body cell can cause cancer, but it is not passed on. A mutation in a gamete, or in a cell that makes gametes, can be inherited.

The effects of mutations

The effect depends on where the mutation is and how it changes the protein.

  • Most: no effect on the phenotype

    Most DNA does not code for proteins, so many mutations happen in non-coding DNA. Also, different triplets can code for the same amino acid, so some changes in a gene make no difference.

  • Some: a small effect

    A mutation can change one amino acid, so a protein works slightly differently. For example, a small change in hair or eye colour.

  • A few: a large effect

    A single mutation can change the phenotype significantly, if it stops an important protein working or makes it work in a new way.

  • Rarely beneficial

    Occasionally a mutation helps an organism survive, such as a bacterium that becomes resistant to an antibiotic.

Mutations in different cells

Mutation in a body cell

  • Happens in a cell such as the skin
  • Can be copied into the new cells made by mitosis
  • May cause cancer if it affects the control of cell division
  • Is not passed on to the next generation

Mutation in a gamete

  • Happens in a cell that makes gametes, or a gamete
  • Is present in every cell of the offspring
  • May cause an inherited disorder or a new variation
  • Is passed on to the next generation

Variation in a population

Over a long time, mutations produce a very large number of different alleles in a species.

  • Extensive variation

    In any species there is usually a lot of genetic variation, because new mutations have been occurring for millions of years.

  • Mixed up in each generation

    Sexual reproduction mixes the alleles into new combinations, so each individual is unique.

  • Neutral variants

    Many variants have no effect now, but they could be useful if the environment changes.

  • Raw material for evolution

    Natural selection can only act on variation that is already present, so mutations are what allow species to evolve.

Case study

Antibiotic resistance in bacteria

Bacteria reproduce very quickly, and a population contains millions of cells. Occasionally a mutation in one bacterium gives it resistance to an antibiotic. Usually this makes no difference, because the bacterium does not meet the antibiotic. But when the antibiotic is used, the non-resistant bacteria are killed and the resistant bacterium survives and reproduces, so the whole population soon becomes resistant. It shows how a single rare mutation can have a large effect.

Rapid reproduction Gives many chances for mutations to occur.
One mutation Can make a large difference to survival.

Summary and exam focus

  • A mutation is a random change in the base sequence of DNA.
  • Ionising radiation and some chemicals increase the rate of mutation.
  • Most mutations have no effect on the phenotype, some have a small effect and a few have a large effect.
  • Mutations in gametes can be inherited, and mutations in body cells cannot.
  • Mutations are a source of new alleles, and so of genetic variation in a population.

Exam focus

Explain why most mutations do not affect the phenotype, and explain why a population of a species usually has a lot of genetic variation. (4 marks) (4 marks)

Give two reasons for mutations having no effect, such as non-coding DNA and the same amino acid being coded by different triplets. For variation, say that mutations happen at random over many generations, and that they produce new alleles.

Key terms

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

Mutation
A random change in the base sequence of DNA.
Allele
A different version of the same gene.
Mutagen
Something that increases the rate of mutation, such as ionising radiation or certain chemicals.
Ionising radiation
Radiation, such as X-rays and gamma rays, that can damage DNA.
Genetic variation
Differences in the genes of individuals in a species.
Phenotype
The observable characteristics of an organism.
Neutral mutation
A mutation that has no effect on the phenotype.
Inherited
Passed from parent to offspring in the genes.
Antibiotic resistance
The ability of bacteria to survive an antibiotic that would normally kill them.

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