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Variation and Natural Selection: Question 3

Syllabus 18.1

Structured Extended 6 marks

A laboratory population of red flour beetles (Tribolium castaneum) is normally reddish-brown in colour. A researcher notices that a small number of beetles in one culture are jet black instead.

(a) Describe what is meant by a gene mutation, with reference to the base sequence of DNA, and state whether such changes normally occur in a directed or a random way. [2]

(b) State two factors that can increase the rate at which mutations occur. [2]

(c) Mutation is not the only source of genetic variation in a population. State two processes, other than mutation, that also produce genetic variation between individuals of a species. [2]

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Worked solution

Part (a): What is a gene mutation?

A gene mutation is a random change in the base sequence of DNA within a gene. The order of bases along a length of DNA determines the protein that gene codes for, so a change to that base sequence can change the protein produced and, in this case, could change the pigment made in the beetle’s body, giving the black colouration instead of the usual reddish-brown. Crucially, mutations happen randomly: they are not produced because the beetle “needs” a new colour, and most mutations arise by chance during DNA replication, entirely independent of whether the resulting change will be useful.

Part (b): Factors that increase the rate of mutation

Two recognised factors that increase the rate of mutation are:

  • ionising radiation, such as X-rays, gamma rays or ultraviolet radiation, which can damage DNA directly
  • certain chemicals (mutagenic chemicals), which can also react with DNA and increase the chance of errors in the base sequence

Either of these two factors is an acceptable answer, and any other correctly named mutagen (for example, a specific ionising radiation type) would also be credited.

Part (c): Other sources of genetic variation besides mutation

Mutation creates entirely new alleles, but a population’s genetic variation also comes from processes that shuffle existing alleles into new combinations:

  • meiosis, during which independent assortment of chromosomes and crossing over between homologous chromosomes produce gametes with new combinations of alleles
  • random mating and random fertilisation, since which two parents mate, and which of the many possible sperm and egg combinations fuse at fertilisation, are both matters of chance, giving each offspring a different combination of its parents’ alleles

Final answers

  • (a) A gene mutation is a random change in the base sequence of DNA within a gene; such changes occur randomly, not in response to need.
  • (b) Ionising radiation (for example X-rays, gamma rays or ultraviolet radiation) and certain chemicals (mutagenic chemicals).
  • (c) Meiosis (independent assortment and crossing over) and random mating together with random fertilisation.