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Unit 4: Maintaining life

Constructing keys — HIA International - Grade 8 Revision Notes & Topical Questions

5 min read 8 parts Lesson 6 of 10 2 quizzes

📚 HIA International - Grade 8 Hub / Topical Revision
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Lesson summary

Constructing keys — HIA INTERNATIONAL notes: Construct and improve a dichotomous key for a set of organisms.Clear difference — each question must split the group using an observable feature.Poor questions — “Is the plant tall?” and “Does it have dark green leaves?” are unclear be…

  • Good questions
  • Branching key
  • Numbered key
  • Questions needed
  • Assessment criteria
  • Balance — each question should split the group into two smaller groups, ideally about equal

Part 1 of 6Choosing questions

Clear difference
  • Clear difference each question must split the group using an observable feature.
Poor questions
  • “Is the plant tall?” and “Does it have dark green leaves?” are unclear because they depend on opinion or vary.
Good questions
  • “Do the flowers have five or more petals?” can be counted.
Balance
  • Balance each question should split the group into two smaller groups, ideally about equal.

Part 2 of 6Steps to construct a key

1
Think of a way to split all the organisms into two groups, e.g. male or female.
2
Take one group and split it into two using another feature, e.g. hair colour.
3
Repeat for each smaller group until every organism is alone.
4
Write the key in a branching or numbered style.
5
Check that the key works for each organism.

Part 3 of 6Writing in the two styles

Branching key
  • Branching key yes/no answers lead along lines to the next question or name.
Numbered key
  • Numbered key pairs of statements 1a/1b, 2a/2b, each ending in “go to” a number or a name.
Limit
  • Limit for a short key use no more than three pairs of statements.
Questions needed
  • Questions needed n organisms need n − 1 questions.

Part 4 of 6Improving a key

1
Give your key to a partner to try out.
2
Ask whether it is easy to choose between statements each time.
3
Use their feedback to change any unclear feature.
4
Test it again.
  • Assessment criteria
    • written as pairs of statements, easy to choose, no more than three pairs, and the key works.

Part 5 of 6Examples of organisms to key

Learners
  • Learners split by male or female, then by a feature such as hair colour.
Cats
  • Pallas’s cat, leopard, lynx and tiger can be keyed by features such as spots or stripes.
Fish
  • Fish stripes, vertical stripes and spines on the fins separate horn shark, zebra fish, dragon fish and clown fish.

Part 6 of 6Keys and classification

Keys and classification
Variation
  • Variation keys use differences in features between species.
Groups
  • Groups they can separate groups such as vertebrates and invertebrates, or insects (6 legs) from arachnids (8 legs).

Key terms

TermMeaning
KeyA set of questions for identifying organisms.
DichotomousBranching into two.
Observable featureA feature you can see or measure.
Branching keyA key drawn as a tree with yes/no lines.
Numbered keyA key written as pairs of statements 1a and 1b.
FeedbackComments used to improve a key.

Avoid these mistakes

1
A question may use any feature — it must be observable with a clear answer.
2
A key can have a question with three answers — a dichotomous key gives exactly two.
3
A key works if only I can follow it — someone else must be able to use it.
4
Keys never need testing — test and improve them with feedback.

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