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Variation

Paper 1Paper 2Paper 3Paper 4

This section is examined in Paper 1, Paper 2, Paper 3, and Paper 4.

What is Variation?
Variation refers to the differences between individuals of the same species. It is crucial to remember that variation occurs within a population, not between different species. For example, humans (same species) vary in height, eye colour, and blood group.

There are two main types of variation:

  1. Continuous Variation: Results in a range of phenotypes between two extremes.
  2. Discontinuous Variation: Results in a limited number of distinct phenotypes with no intermediates.
Continuous Variation
Continuous variation is variation where the phenotype falls anywhere along a continuous scale. There are no distinct categories; instead, there is a smooth gradation from one extreme to the other.

Causes: It is caused by both genes and the environment. Multiple genes (polygenic inheritance) interact with environmental factors to produce the final phenotype.

Examples of Continuous Variation in Humans:

  • Body mass
  • Height
  • Hand span

Examples of Continuous Variation in Plants:

  • Leaf area
  • Plant height
Discontinuous Variation
Discontinuous variation is variation where the phenotype falls into distinct, separate categories. There are no intermediates between these categories.

Causes: It is usually caused by genes only. The trait is typically controlled by a single gene or a small number of genes, with little to no environmental influence.

Examples of Discontinuous Variation in Humans:

  • ABO blood groups (A, B, AB, O)
  • Ability to roll the tongue
  • Earlobe attachment (attached vs. free)

Examples of Discontinuous Variation in Plants:

  • Seed shape in peas (round vs. wrinkled)
  • Seed colour in peas (yellow vs. green)
Mutation and Genetic Change
Mutation is defined as a genetic change. It is the primary way in which new alleles are formed. Without mutation, there would be no new genetic variation for natural selection to act upon.
Gene Mutation (Supplement): This is a random change in the base sequence of DNA. A change in the DNA base sequence can alter the amino acid sequence of a protein, potentially changing the phenotype.

Factors Increasing Mutation Rate: The rate of mutation can be increased by:

  1. Ionising radiation (e.g., X-rays, gamma rays, UV light).
  2. Some chemicals (mutagens).
Sources of Genetic Variation in Populations

Genetic variation within a population arises from four main sources:

  1. Mutation: Creates new alleles.
  2. Meiosis: The process of gamete formation introduces variation through crossing over and independent assortment.
  3. Random Mating: Individuals choose mates randomly, mixing gene pools.
  4. Random Fertilisation: Any sperm can fertilise any egg, combining genetic material in unique ways.
Investigating Variation: Describing the Results
When investigating variation, you must describe the pattern of results observed. The command word 'describe' requires you to state what the data shows, not just how you collected it.
Type of Variation InvestigatedExpected Graph/PatternDescription of Results
Continuous Variation (e.g., Height)Histogram or Line GraphThe results show a bell-shaped curve (normal distribution). There is a range of phenotypes between two extremes. Most individuals are near the average, with fewer at the extremes.
Discontinuous Variation (e.g., Blood Group)Bar ChartThe results show distinct categories (peaks) with no intermediates. The data falls into separate groups (e.g., only Type A, B, AB, or O). There are no values between the categories.
Why this matters: If you investigate height and get a bell curve, you describe it as continuous variation caused by genes and environment. If you investigate blood groups and get distinct bars, you describe it as discontinuous variation caused by genes only.
⚠︎ Confusing Variation Types and Causes
Mistake 1: Believing that seed colour in peas is an example of continuous variation.
Correction: Seed colour in peas is discontinuous. It is either yellow or green, with no intermediate shades. This is controlled by genes only.
Mistake 2: Believing that ABO blood groups are an example of continuous variation.
Correction: Blood groups are discontinuous. You have one specific group (A, B, AB, or O). There is no 'in-between' blood type.
Mistake 3: Thinking that variation caused by the environment can be inherited.
Correction: Only changes in the DNA (genetic) material are inherited. If a person gets a tan from the sun (environmental change), their offspring will not be born with a tan. This is an acquired characteristic, not genetic variation.
Mistake 4: Forgetting that meiosis is a source of genetic variation.
Correction: Meiosis creates genetically unique gametes through crossing over and independent assortment. It is a key source of variation alongside mutation.
Describing Variation in Investigations
Context: When asked to 'describe' the results of an investigation into variation (e.g., height or blood groups).

Examiner Acceptance: Examiners look for specific keywords related to the distribution of the data. For continuous variation, you must mention a 'range' or 'bell-shaped curve'. For discontinuous variation, you must mention 'distinct categories' or 'no intermediates'.

Example: 'The results for height show a range of values forming a bell-shaped curve, indicating continuous variation. The results for blood groups show distinct peaks with no individuals in between, indicating discontinuous variation.'

Reasoning: This directly addresses the definition of the variation types by describing the phenotypic distribution observed in the sample.

Context: When asked to define 'mutation' or 'gene mutation'.

Examiner Acceptance: For 'mutation', use 'genetic change'. For 'gene mutation', you must include 'random' and 'change in the base sequence of DNA'.

Example: 'Gene mutation is a random change in the base sequence of DNA.'

Reasoning: The word 'random' is essential because mutations occur by chance, not in response to need. 'Base sequence' is more precise than just 'DNA structure'.

Past Paper Style Questions
Q:
Define the term variation. [2]
A:
Differences between individuals; of the same species.
Q:
State two examples of continuous variation in humans. [1]
A:
Any two from: height, body mass, hand span, skin colour.
Q:
State one example of discontinuous variation in plants. [1]
A:
Seed shape in peas OR seed colour in peas.
Q:
State the type of variation shown by ABO blood groups and explain your answer. [2]
A:
Discontinuous; because there are distinct categories with no intermediates.
Q:
Describe how mutation leads to new alleles. [1]
A:
Mutation is the way in which new alleles are formed.
Q:
State two factors that can increase the rate of mutation. [2]
A:
Ionising radiation; (named) chemicals.
Q:
Describe gene mutation. [2]
A:
Random change in the base sequence of DNA.
Q:
State four sources of genetic variation in populations. [4]
A:
Mutation; meiosis; random mating; random fertilisation.
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