A3.1.14 (HL)—Dichotomous key development
Dichotomous keys identify organisms through paired observable choices that separate groups step by step until one taxon or specimen remains.
- Syllabus
- First assessment 2025
- Objective
- A3.1.14
- Level
- HL
Exam analysis
Chance of appearing1%of analysed past papers
Latest appearanceMay 2014
Most common paperPaper2
Typical marks1
Common command terms
- Identify
Scoring notes
- Common mistake
- Choosing an organism from appearance alone instead of following each key branch.
Recent exam appearances
May 2014Paper2 ["HL"] · TZ23(a)[ 1 ]A3.1.14 (HL)—Dichotomous key development
Practice this objective
Coverage 2014–2014 · Updated 15 Jul 2026
Dichotomous keys and DNA identification
HL only
Choose the identification tool that matches the available evidence.
- Dichotomous key: repeatedly choose between paired statements based on observable traits. Use reliable, relatively immutable features such as structures or reproductive processes; size, colour and behaviour may vary with age or environment. Each choice should lead to one narrower group until the specimen is identified.
- DNA barcoding: amplify and sequence a short standard region of DNA from a specimen, then compare it with a reference database to identify the species.
- Environmental DNA (eDNA): collect DNA traces from water, soil or another habitat, amplify barcode regions by PCR and match sequences to a database; it can detect organisms that are not directly observed.
Keys are useful when visible features are intact and reliable. Barcoding or eDNA is more useful when specimens are damaged, partial, cryptic or absent. Local plant or animal species can be used to build and test a key, but a database match is evidence of sequence similarity, not automatically proof that every individual in the habitat was sampled.
Dichotomous key development
HL onlyAssessment in practice
1 marksHow it is assessed
This objective is assessed through structured response, commonly using Identify.
Command terms
Identify
What earns marks
Build the answer around this relationship: Dichotomous keys use paired contrasting choices.
Watch for
Choosing an organism from appearance alone instead of following each key branch.
Representative question
Question 1
Parts of a dichotomous key to organisms A, B, C and D are shown. Design missing parts of the key using features visible in the following diagrams.
1. Body with tentacles ..... A
Body without tentacles ..... go to 2
2. ..... B
……go to 3
3. ..... C
.....D
Boundaries And Identification
HL onlyThe HL extension connects two problems: species boundaries and species identification. Bacteria challenge the biological species concept because they are asexual and exchange genes horizontally. Chromosome number can help define boundaries in many sexual species because meiosis conserves it and mismatches can produce infertile hybrids. Dichotomous keys identify visible organisms using paired choices. DNA barcoding and eDNA identify species using short sequences, PCR, and databases when visible traits are limited.
- Bacterial boundaries are blurred by mutation and horizontal gene transfer.
- Chromosome number is usually conserved within sexual species by meiosis.
- Different chromosome numbers can prevent fertile hybrids, as in mules.
- Dichotomous keys use observable traits and paired choices.
- DNA barcoding/eDNA use PCR and database comparison for damaged, partial, hidden, or difficult specimens.
- Local plant or animal species can be used to build and test dichotomous keys.
Concept essentials
- Dichotomous keys use paired contrasting choices.
- Each answer in a key leads to another choice or a final identification.
- Useful key characters must be observable and able to separate organisms clearly.
- Constructing a key requires comparing organisms feature by feature.