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2.3Biology 02. Life Processes

Use and construct dichotomous keys — Cambridge Checkpoint Science 0893 Revision Notes & Topical Questions

5 min read 8 parts Lesson 3 of 13 1 quiz

📚 Cambridge Checkpoint Science 0893 Hub / Topical Revision
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Lesson summary

2.3 Dichotomous keys: using and constructing them — CHECKPOINT - Lower Secondary Science 0893 notes: Use a dichotomous key to identify organisms and construct a dichotomous key to separate a set of organisms.Key — a set of questions about an organism, where each answer leads to a…

  • Pairs of statements
  • Following the key
  • Clear features
  • Poor features
  • Key — a set of questions about an organism, where each answer leads to another question until the name is reached
  • Dichotomous key — a key in which every question has two possible answers, meaning it branches into two

Section 1 of 8What a key is

Key
  • Key a set of questions about an organism, where each answer leads to another question until the name is reached.
Dichotomous key
  • Dichotomous key a key in which every question has two possible answers, meaning it branches into two.

Section 2 of 8Using a branching key

1
Choose one organism to identify.
2
Start at the top of the key and answer the first question with yes or no.
3
Follow the line to the next question.
4
Keep going until you reach the name of the organism.

Section 3 of 8Using a key written as numbered pairs of statements

Pairs of statements
  • Pairs of statements each number gives two statements, labelled a and b, to choose between.
  • Each statement leads to another number or to the name of the organism.
Following the key
  • Following the key work through the pairs in the order the numbers tell you.
  • To reach dragonfly the steps are 1a, 2a and 4a.

Section 4 of 8Features suitable for a key

Features suitable for a key
Clear features
  • Clear features a question must be about a feature that can be seen and answered yes or no.
  • Good examples are the number of legs, the presence of wings or whether the body has rings.
Poor features
  • Poor features vague features are not good because different people judge them differently.
  • Examples are "is the plant tall?" and "does it have dark green leaves?".

Section 5 of 8Constructing a key

1
Split the whole set of organisms into two groups using one clear feature.
2
Take one group and split it into two using another feature.
3
Repeat step 2 for each group until every organism is separated.
4
Write the key as branching questions or as numbered pairs of statements.
5
Test the key on another person and improve it.

Section 6 of 8Checking a good key

Checking a good key
Each choice is easy to make.
Each pair of statements separates organisms using clear features.
The key works every time and identifies every organism.

Section 7 of 8Key terms

TermMeaning
KeyA set of questions used to identify an organism.
DichotomousBranching into two.
IdentifyTo find the name of an organism.
ClassifyTo place organisms into groups.

Section 8 of 8Exam technique

Avoid these mistakes
A key can have three choices at each step — a dichotomous key always has exactly two choices.
Any feature can be used in a key — features must be clear, such as number of legs, not size words like tall.
A key does not need testing — testing by another user finds and fixes problems.

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