Investigating diversity
AQA
A-Level
Genetic diversity is compared using four methods. Originally, scientists measured the frequency of observable characteristics. However, most traits are polygenic and modified by the environment, making phenotypes an indirect guide to genotypes. Modern methods compare molecules directly. Comparing DNA base sequences is the most direct; fewer differences indicate closer relatedness. Comparing mRNA base sequences works similarly, as mRNA is complementary to the DNA template strand. Finally, comparing the amino acid sequences of proteins encoded by DNA and mRNA also reveals diversity. Because the genetic code is degenerate, amino acid sequences may underestimate DNA differences, but they still provide clear evidence of evolutionary relationships.
9
Objectives
7
Exam Tips
14
Pitfalls
14
Key Terms
15
Mark Points
Subtopics in this area
Genetic diversity within, or between species, can be made by comparing: the frequency of measurable or observable characteristics the base sequence of DNA the base sequence of mRNA the amino acid sequence of the proteins encoded by DNA and mRNA.
Students should be able to: interpret data relating to similarities and differences in the base sequences of DNA and in the amino acid sequences of proteins to suggest relationships between different organisms within a species and between species appreciate that gene technology has caused a change in the methods of investigating genetic diversity; inferring DNA differences from measurable or observable characteristics has been replaced by direct investigation of DNA sequences. Knowledge of gene technologies will not be tested.
Quantitative investigations of variation within a species involve: collecting data from random samples calculating a mean value of the collected data and the standard deviation of that mean interpreting mean values and their standard deviations.
Investigating diversity Revision Guide
Learning Objectives
What you need to know and understand
- Compare two DNA or amino acid sequences and state what the number of differences shows about genetic diversity.
- Explain why mRNA base sequences and amino acid sequences can stand in for DNA sequences in such comparisons.
- Justify why comparing observable characteristics is a less reliable measure of genetic diversity than molecular sequencing.
- Rank organisms by relatedness from a table of sequence differences and justify the ranking.
- Explain why direct DNA sequencing replaced inference from observable characteristics when investigating genetic diversity.
- State one limitation of drawing conclusions about a whole species from a single gene or a small sample.
- Describe how to collect a random sample of individuals from a population and explain why it avoids bias.
- Interpret means and standard deviations, recognising that overlapping SDs indicate a difference is likely not significant.
- Evaluate investigations by identifying when a statistical test is required to confirm whether a difference is significant.
Marking Points
Key points examiners look for in your answers
- Naming the base sequence of DNA, the base sequence of mRNA, or the amino acid sequence of a protein as the comparison made.
- Stating that the more similar the sequences, the smaller the genetic diversity or the closer the evolutionary relationship.
- Explaining that mRNA sequences reflect DNA sequences because mRNA is transcribed from the DNA template strand.
- Explaining that the DNA base sequence determines the amino acid sequence, allowing proteins to be compared.
- Identifying the limitation that observable characteristics are polygenic and influenced by the environment.
- one mark for comparing the sequences given and stating which pair differs least
- one mark for concluding that fewer differences indicate a closer relationship or a more recent common ancestor
- one mark for expressing a comparison within a species as genetic diversity between populations rather than as separate species
- one mark for stating that DNA sequences are now investigated directly rather than inferred from observable characteristics
- one mark for a limitation, such as one gene or a small sample not being representative of the whole genome or population
- Describe random sampling as ensuring every individual has an equal chance of selection, which avoids investigator bias.
- State that a large sample size or using repeat samples reduces the effect of chance and makes the sample more representative of the population.
- Explain that overlapping standard deviation bars indicate the difference between means is likely not significant and could be due to chance.
- State that non-overlapping standard deviation bars suggest a significant difference, but explicitly note that a statistical test is required to confirm significance.
- Identify that standard deviation measures the spread of data around the mean, rather than being a statistical test itself.
Examiner Tips
Expert advice for maximising your marks
- 💡Always write 'base sequence' or 'amino acid sequence'; the unqualified word DNA or protein is insufficient for marks.
- 💡If a question provides sequence data, count the exact number of differences and explicitly state what this count means about relatedness.
- 💡Write the count and the conclusion together: 'only two differences, so these two are the most closely related'.
- 💡Keep answers about the change in method at the level of principle, since direct sequencing has replaced inference from phenotype.
- 💡Where a question compares members of one species, use the words genetic diversity rather than speciation.
- 💡When evaluating data with error bars, always state whether the standard deviations overlap before commenting on the likelihood of the difference being significant.
- 💡If asked to evaluate a conclusion based only on means and standard deviations, point out that a statistical test has not been carried out to confirm significance.
Common Mistakes
Pitfalls to avoid in your exam answers
- Writing 'compare the DNA' without specifying that it is the base sequence being compared; correct by always stating 'base sequence'.
- Forgetting that the genetic code is degenerate, meaning two different base sequences can produce the same amino acid sequence; correct by noting this limitation.
- Stating mRNA is identical to DNA rather than complementary to the template strand; correct by specifying it is complementary with uracil replacing thymine.
- Treating observable characteristics as a direct readout of the genotype; correct by explaining the role of the environment and polygenic inheritance.
- Comparing different proteins between two species instead of comparing the same protein; correct by specifying the comparison of homologous proteins.
- concluding that two organisms are the same species because their sequences are similar
- quoting the number of differences without saying what it means for relatedness
- describing sequencing techniques such as PCR in detail, which is not credited in this section
- assuming the organism whose sequence is printed first is the ancestor of the others
- ignoring the possibility that one gene gives a different answer from the rest of the genome
- Stating that non-overlapping standard deviations prove a difference is statistically significant. Correction: State that non-overlap only suggests significance; a statistical test is required to confirm it.
- Describing random sampling as picking individuals 'at random' by eye. Correction: Recognise that picking by eye introduces investigator bias; true random sampling requires an objective method like a random number generator.
- Confusing standard deviation with a statistical test. Correction: Remember that standard deviation is a measure of data spread around the mean, whereas a statistical test determines the probability that a difference is due to chance.
- Interpreting a large standard deviation as an error in the experimental method. Correction: Understand that a large standard deviation reflects a wide natural variation or spread of values within the sampled population.