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1.6Characteristics and Classification of Living Organisms

Dichotomous keys — Cambridge IGCSE Biology 0610 Revision Notes & Topical Questions

5 min read 9 parts Lesson 6 of 6 2 quizzes

📚 Cambridge IGCSE Biology 0610 Hub / Topical Revision
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Lesson summary

1.6 Dichotomous keys — IGCSE BIOLOGY notes: Use and construct dichotomous keys based on visible features.Dichotomous key — A tool for identifying organisms using a series of questions about their features.Dichotomous — Means branching into two.At each step two descriptions are gi…

  • Each organism
  • Steps needed
  • Statements
  • Branching key
  • Observable feature
  • Classification

Part 1 of 7What a dichotomous key is

Dichotomous key
  • A tool for identifying organisms using a series of questions about their features.
Dichotomous
  • Means branching into two.
  • At each step two descriptions are given and one is chosen.
  • Each choice leads on to another pair of descriptions or to a name.

Part 2 of 7Using a branching key

1
Use the information or image given to answer the first statement.
2
Follow the statement chosen to the next pair of statements.
3
Keep working through the key until no statements are left on the branch followed.
4
Read off the organism identified at the end of the branch.
5
Return to the start of the key for each further organism and repeat.

Part 3 of 7Using a table key

Format
  • Each numbered pair of statements either says "go to" another number or gives an organism name.
Method
  • Begin at statement 1, choose the description that fits, and follow the instruction it gives.
Each organism
  • Start again at statement 1 for every organism to be identified.

Part 4 of 7Constructing a key

1
List the organisms and note their visible features.
2
Choose a feature that divides the group into two smaller sets.
3
Write two opposite statements about that feature.
4
Repeat on each smaller set until every organism is separated.
5
Check that the key leads to each organism, using each organism in turn.

Part 5 of 7Good features for a key

Good featurePoor feature
Number of legs, which can be countedSize or age, which varies
Wings present or absent, a clear yes or noColour alone, which can differ within a species
Body in two or three parts, which is fixedHabitat, which is not a body feature

Part 6 of 7Key facts

1Steps needed
  • A key separating n organisms needs n − 1 questions.
2Statements
  • The two statements in a pair must describe opposite states of the same feature.
3Exam use
  • Questions usually ask you to use a key, but the syllabus also asks you to construct a simple one.

Part 7 of 7More from the specification

1
Mammalian classification and mammary glands
  • Mammalian classification and mammary glands presence of insulating hair and nutrient-supplying mammary glands distinguishes class Mammalia from other vertebrate taxa.
2
Biological Classification System
  • Living organisms are organized into hierarchical taxonomic groups using the universal binomial naming system.
    • (kingdom, phylum, class, order, family, genus, species)
3
Structure and Use of Dichotomous Keys
  • A dichotomous key consists of a sequence of paired, contrasting statements based on observable morphological characteristics that lead the user to identify an unknown organism.
4
Identifying Plants Using Leaf Features
  • Keys for terrestrial plants frequently employ vegetative characteristics such as leaf venation, leaf margin shape,
  • and whether leaves are simple or compound.
5
Modern Classification and DNA Sequences
  • While traditional classification relied on observable characteristics of cells and anatomy, modern phylogenetic classification relies on comparing base sequences of DNA and amino acid sequences in proteins.
6
Binomial
  • A two-part Latin name of genus then species.
7
Binomial system
  • An internationally agreed system naming each organism with a genus and a species in Latin.
8
Mammary gland
  • A gland producing milk for young.
9
Classification
  • Grouping organisms by shared features and relationships.
10
Sequence
  • The order of bases in DNA or amino acids in a protein.
11
Monocotyledon
  • A flowering plant with one cotyledon, parallel leaf veins and petals in threes.
12
Dicotyledon
  • A flowering plant with two cotyledons, reticulated leaf veins and petals in fours or fives.
13
Kingdom
  • One of the five largest groups into which living organisms are divided.
14
Multicellular
  • Made of many cells.
15
Nucleus
  • The part of the cell that controls its activities.
16
Nutrition
  • Taking in materials for energy, growth and development.

Key terms

TermMeaning
Dichotomous keyA tool that gives paired descriptions leading step by step to the identity of an organism.
DichotomousBranching into two.
Branching keyA key shown as a diagram that splits into two at each step.
Table keyA key written as numbered pairs of statements that direct the user to another number or a name.
Observable featureA feature that can be seen or counted without special tests.

Avoid these mistakes

1
Starting a later organism part-way through the key — begin at the first statement every time.
2
Writing two statements about different features — a pair must be opposites of the same feature.
3
Using features that cannot be seen — use observable features such as legs, wings or shell.
4
Giving a vague feature such as big or dark — use features that can be counted or clearly seen.

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