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LESSON C5 · EXPLAINER
Genetic Variation
Measure the hair length of everyone in your class and you’ll get a smooth range. Ask who is left-handed and you get two groups. Why the difference?
Narration uses your device’s built-in voice. No sound? Switch to “Captions only” — the captions advance on their own.
Step 2 · Read
Key notes
Copy these into your notebook in your own words — that is how they stick.
Genetic variation
Differences in genotype between individuals of a species.
Sources
Crossing over, independent assortment, random fertilisation, mutation.
Mutation
Random change in DNA sequence or chromosome number; source of new alleles.
Importance
Survival in changing conditions; basis for natural selection and speciation.
Continuous
Range of values; many genes + environment; histogram (e.g. height, mass).
Discontinuous
Distinct categories; one/few genes; bar chart (e.g. blood group, handedness).
Asexual reproduction
Produces NO genetic variation (except mutation).
EXAM TIPS & COMMON MISTAKES
Right/left-handedness = DISCONTINUOUS variation → bar chart.
Variation “is due to mutation and crossing over” — NOT asexual reproduction.
When plotting, choose a scale that uses at least half the grid and plot points accurately.
Step 2 · Learn from examples
Worked examples & study resources
The kind of support a teacher gives after the lesson: worked examples to attempt first, a board illustration, key terms, common misconceptions and memory aids.
Worked example 1
Continuous or discontinuous?
Classify each characteristic and give a reason: height; ABO blood group; ability to roll the tongue; body mass; leaf length; number of petals on a hibiscus flower.
Try it in your notebook first, then reveal the solution one step at a time.
Ask: can it take ANY value in a range (continuous), or does it fall into distinct groups (discontinuous)?
Continuous: height, body mass, leaf length — measured on a scale, with many intermediate values.
Discontinuous: blood group (A, B, AB, O), tongue rolling (can / cannot), number of petals (whole numbers in fixed categories).
Continuous variation is usually controlled by many genes AND affected by the environment; discontinuous variation is usually controlled by one or a few genes, with little environmental effect.
Answer. Continuous: height, body mass, leaf length. Discontinuous: blood group, tongue rolling, number of petals.
Examiner’s tip. Graph type is a classic follow-up: continuous data → histogram (bars touching); discontinuous data → bar chart (bars separate).
Worked example 2
Reading variation data
Heights of 39 students:
Height (cm)
140–144
145–149
150–154
155–159
160–164
165–169
Number
2
5
9
12
8
3
(a) Which graph should be drawn? (b) Name the modal class. (c) Describe the shape of the distribution. (d) Suggest why height varies.
Try it in your notebook first, then reveal the solution one step at a time.
(a) Height is continuous, so draw a histogram with height on the x-axis and number of students on the y-axis; bars touch.
(b) The class with the most students is 155–159 cm (12 students).
(c) Most students are near the middle with fewer at the extremes — a roughly bell-shaped, normal distribution.
(d) Height is affected by many genes (genetic variation) and by the environment, such as diet and health during childhood.
Answer. (a) Histogram. (b) 155–159 cm. (c) Roughly normal (bell-shaped) distribution. (d) Many genes plus environmental factors such as diet.
Examiner’s tip. A bell-shaped histogram is the signature of continuous variation — say so if the question asks what the graph shows.
On the board · Continuous and discontinuous variation
Continuous
Discontinuous
Values
Any value within a range
Distinct categories, no in-betweens
Controlled by
Many genes + environment
One or a few genes
Environmental effect
Large
Little or none
Graph
Histogram (normal distribution)
Bar chart
Examples
Height, mass, leaf length, hand span
Blood group, tongue rolling, sickle cell trait
One table answers most “distinguish between” questions on this topic.
Key terms
Variation
Differences between individuals of the same species.
Genetic variation
Differences caused by different alleles (inherited).
Environmental variation
Differences caused by conditions such as diet, light or climate.
Mutation
A random change in a gene or chromosome — the original source of new alleles.
Continuous variation
Variation with a full range of values between two extremes.
Discontinuous variation
Variation in distinct groups with no intermediates.
Common misconceptions
✗ All variation is inherited.✓ Much variation is caused by the environment, and many features are a mix of both.
✗ Identical twins are identical in every way.✓ They share the same genes, but environment (diet, exercise, injury) makes them differ.
✗ Mutations are always harmful.✓ Most have no effect, a few are harmful, and some are beneficial — they provide the variation natural selection acts on.
Memory aid
Continuous = Can be measured on a scale. Discontinuous = Distinct groups.
Real-life link
The sugar cane breeding station in Barbados has bred new sugar cane varieties for the Caribbean for over a century, selecting from the natural genetic variation among thousands of seedlings for yield, sugar content and disease resistance.
Try this
Measure the hand span of 20 people. Group the results into classes, draw a histogram and describe its shape.
Play & practise
Games
3 quick games on this lesson. Earn XP and badges as you play — your best score for each game is saved. Games are for practice and do not change your exam readiness.
Step 3 · Practise
Paper 1 practice — multiple choice
8 items modelled on real CSEC Paper 1 questions. Choose one answer for each, then check. Aim for 80% or better.
Step 4 · Apply
Paper 2 practice — structured questions
Write full answers, then reveal the mark scheme and tick the points you earned (16 marks in total). Your answers are saved on this device.
Based on CSEC P2 January 2022, Question 1(a)–(d)
Question 1 [12 marks]
Sarah measured hair length of 16 girls: 2 in (1 girl), 4 in (4), 5 in (2), 7 in (8), 10 in (1).
(a) Describe how you would plot these results (axes, scale, graph type, title). [5]
MARK SCHEME — (a) (max 5)
(b) State TWO precautions for this investigation. [2]
MARK SCHEME — (b) (max 2)
(c) Define continuous and discontinuous variation, and state the type shown by right/left-handedness. [3]
MARK SCHEME — (c) (max 3)
(d) Suggest ONE way genetic variation in moth wing colour arose and ONE reason it is important to the moths. [2]
MARK SCHEME — (d) (max 2)
Based on CSEC P2 May/June 2024 Q2(b) and January 2024 Q5(d)
Question 2 [4 marks]
Genetic variation in populations.
(a) List TWO ways genetic variation occurs in living organisms. [2]
MARK SCHEME — (a) (max 2)
(b) Explain why a crop with no genetic variation could be destroyed by a single disease. [2]
MARK SCHEME — (b) (max 2)
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Sources
Where this topic appears in CSEC papers
Find these in your own past-paper booklets for extra timed practice. The practice above is adapted from them.
CSEC P1 May/June 2022, 2023, 2024 — Item 57 (sources of variation); Item 52/53 (no variation in sugar cane)
CSEC P2 January 2022 — Question 1 (hair-length graph; continuous vs discontinuous; moths)