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Investigating diversityAQA A-Level Biology: Revision notes

Section 1

Genetic diversity and how it is investigated

Genetic diversity is the variety of alleles and genes within a species or between species. It can be investigated by comparing, within or between species:

  • the frequency of measurable or observable characteristics, such as height, flower colour or fur colour
  • the base sequence of DNA
  • the base sequence of mRNA
  • the amino acid sequence of the proteins encoded by the DNA and mRNA.
Key termsgenetic diversity

Section 2

Comparing sequences and relationships

DNA codes for mRNA, which codes for the amino acid sequence of a protein. Mutations change the base sequence over time, and some of these change the protein.

The more differences there are between the sequences of two organisms, the longer ago they shared a common ancestor, so the more distantly related they are. Few differences suggest a close relationship.

Example. The same protein has 1 amino acid difference between humans and rhesus monkeys but 21 between humans and tuna. Humans and rhesus monkeys therefore share a more recent common ancestor.

Key termsmutationcommon ancestor
Exam tip

Link the number of differences to time: more differences means more mutations accumulated, so an older common ancestor.

Section 3

Differences hidden in proteins

Comparing DNA gives more information than comparing proteins. The genetic code is degenerate, so a change in a base may not change the amino acid. DNA also contains non-coding sequences that are never translated. These differences are only detected by comparing DNA directly.

Key termsdegenerate codenon-coding DNA
Common mistake

A difference in DNA does not always mean a difference in the protein. Do not state that it always does.

Section 4

Gene technology

In the past, genetic diversity was inferred from observable characteristics. These can be misleading, because they are influenced by the environment, by recessive alleles and by several genes acting together. Gene technology now allows the base sequences of DNA to be compared directly, which is more accurate and objective. You do not need to know how the technologies work.

Key termsgene technology

Section 5

Quantitative investigations of variation

To investigate variation within a species, collect data from random samples, which avoids bias. Calculate the mean and the standard deviation of the measurements. You will not have to calculate a standard deviation, but you must interpret it.

  • A small standard deviation means values are clustered close to the mean.
  • A large standard deviation means there is more variation.
  • If mean ± standard deviation ranges for two samples overlap, the difference between the means may be due to chance.

Larger samples give more reliable means.

Key termsmeanstandard deviationrandom sample

That's the notes covered.

Carry on to the next subtopic.

Exam questions on Investigating diversity

  1. Scientists compared the amino acid sequence of the same protein, cytochrome c, from four species and counted the number of differences from the human sequence. The chimpanzee sequence had 0 differences, the rhesus monkey sequence had 1, the horse sequence had 12 and the tuna sequence had 21.
    Explain why the human cytochrome c sequence has fewer differences from that of the rhesus monkey than from that of the tuna.2 marks
  2. Two individuals of the same beetle species were found to differ by one base in the DNA sequence of the gene coding for an enzyme. When the amino acid sequences of the enzyme in the two beetles were compared, they were identical.
    Suggest why comparing the base sequences of DNA gives a more complete measure of genetic diversity than comparing amino acid sequences.2 marks
  3. A biologist measured the wing length of 50 randomly chosen moths from each of two populations. The mean wing length of population A was 24.2 mm with a standard deviation of 1.1 mm. The mean wing length of population B was 25.0 mm with a standard deviation of 3.2 mm.
    Explain what the standard deviations show about the wing lengths of the two populations.3 marks
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Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).