Compare DNA sequences as classification evidence
Count aligned base differences and explain the limits of one short sequence.
Exam scope and assessment notes
Extended only (Supplement) · Biology 0970, 2026–2028. Use the stated specimen features and evidence. Diagrams are schematic. Written practice is self-checked; human subject-teacher review, supervised practical competence and cumulative assessment remain pending. Cambridge states that IGCSE 0970 has the same subject content as IGCSE 0610, with 9–1 grading. This StudyForA pathway maps the 2026–2028 syllabus only. This 9–1 graded qualification is available only in a limited number of administrative zones. Confirm that your centre can enter you for this code and exam series with Cambridge International. Cambridge has published a separate syllabus for 2029. StudyForA has not reconciled that version yet; this page supports exams in 2026, 2027 and 2028 only.
Before you begin
- Read the stated observations before choosing a group.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
What is being compared in DNA-based classification?
What you will learn
- Use DNA base sequences as evidence.
- Connect greater similarity with closer evolutionary relatedness.
Compare equivalent DNA regions
DNA is made from sequences of bases. Comparing the same region across organisms can provide evidence for classification. Organisms sharing a more recent ancestor generally have more similar corresponding sequences than organisms related only through a distant ancestor.
For the supplied aligned fragments A = ACGTACGTAA, B = ACGTACGTAG and C = TCGTTCGTCC, compare each position from left to right. A and B differ at one of ten positions; A and C differ at four. The smaller difference supports closer A–B relatedness in this exercise.
Evidence is not a complete ancestry proof
The fragments must represent corresponding regions in the same orientation. Comparing arbitrary unrelated pieces, or misaligning positions, can manufacture misleading differences. Count substitutions at each aligned position without skipping repeated letters.
One short fragment is limited evidence. Longer sequences, more genes and structural observations can strengthen an interpretation. This exercise does not date an ancestor, establish a species boundary, or imply that the most similar living organism is the direct ancestor of another.
Pause and explain
A–B differs at one aligned site; A–C differs at four. What is supported?
Worked example
Compare ACGTACGTAA with ACGTACGTAG. Count the matching positions and give the percentage match.
Show the worked solution
- There are ten aligned positions.
- Only position 10 differs, so nine match.
- 9 ÷ 10 × 100 = 90%.
Answer 9 matches; 90% match in this supplied fragment.
Try a topic test
Choose Easy, Normal or Difficult. Save your answers and open worked review after finishing.
Test this topic →From guided practice to a new situation
Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
Count differences between ACGTACGTAA and TCGTTCGTCC.
Give me a hint
Compare positions 1, 5, 9 and 10 carefully.
Compare my reasoning
- Position 1 differs: A/T.
- Position 5 differs: A/T.
- Positions 9 and 10 differ: A/C and A/C.
4 differences; 6 matches.
Look for these in your work
- Count all four substitutions.
- Keep ten aligned positions.
Find the percentage match for that pair of ten-base fragments.
Give me a hint
Use matches divided by total positions.
Compare my reasoning
- Six positions match.
- Divide 6 by 10.
- Multiply by 100.
60% match.
Look for these in your work
- Use six matches.
- State that the percentage applies to this fragment.
One ten-base fragment suggests A–B is closer than A–C. Propose an improvement and one conclusion not justified yet.
Give me a hint
Expand corresponding genetic evidence.
Compare my reasoning
- Compare longer aligned regions or additional genes.
- Check against other relevant evidence.
- Do not claim a dated ancestor or a complete species boundary.
Compare more corresponding DNA; the short fragment alone does not date ancestry or establish species identity.
Look for these in your work
- Give a specific evidence improvement.
- State a concrete inference limit.
Common mistakes
- Compare corresponding aligned regions.
- Short sequence similarity is evidence, not a complete phylogenetic proof.
What can you explain now?
Why should corresponding DNA regions be compared?
Compare with the explanation
Different unrelated regions would not provide a fair position-by-position comparison.
The evidence depends on equivalent aligned sequence positions.
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Make it stick
Count the differences in both supplied pairs and explain why more DNA evidence would help.
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.