CIE iGCSE Biology-18.1 Variation- Study Notes- New Syllabus
CIE iGCSE Biology-18.1 Variation- Study Notes – New syllabus
CIE iGCSE Biology-18.1 Variation- Study Notes -CIE iGCSE Biology – per latest Syllabus.
Key Concepts:
Core
- Describe variation as differences between individuals of the same species
- State that continuous variation results in a range of phenotypes between two extremes; examples include body length and body mass
- State that discontinuous variation results in a limited number of phenotypes with no intermediates; examples include ABO blood groups, seed shape in peas and seed colour in peas
- State that discontinuous variation is usually caused by genes only and continuous variation is caused by both genes and the environment
- Investigate and describe examples of continuous and discontinuous variation
- Describe mutation as genetic change
- State that mutation is the way in which new alleles are formed
- State that ionising radiation and some chemicals increase the rate of mutation
Supplement
- Describe gene mutation as a random change in the base sequence of DNA
- State that mutation, meiosis, random mating and random fertilisation are sources of genetic variation in populations
Variation – Differences Within a Species
What is Variation?
Variation means the differences that exist between individuals of the same species.
Even though members of a species share the same basic features, they are not exactly alike – each individual is slightly different.
🧾 Examples of Variation:
| In Humans | In Plants |
|---|---|
| Height | Leaf shape |
| Eye colour | Flower colour |
| Skin tone | Height of the plant |
| Blood group | – |
| Ability to roll the tongue | – |
Variation helps explain why no two individuals (except identical twins or clones) are exactly the same.
Continuous Variation – A Range with No Gaps
Definition:
Continuous variation is when a characteristic shows a range of phenotypes between two extremes, with no clear-cut categories.
🧾 Key Features:
- Many small differences between individuals
- Measured in numbers (like height or weight)
- Usually affected by many genes and environment
🔍 Examples:
- Body length in animals
- Body mass in humans
- Skin colour
- Leaf size in plants
Continuous variation = smooth range of values, not fixed types
(e.g. people can be any height, not just “short” or “tall”)
Discontinuous Variation – Clear Differences, No In-Between
Definition:
Discontinuous variation is when a characteristic has a limited number of distinct phenotypes, with no intermediates or gradual changes.
In other words, individuals fall into separate groups, not along a smooth range.
🧾 Key Features:
- Controlled by a single gene or a few genes
- Not influenced much by the environment
- Shows up as clear categories
- No blending or halfway traits
📌 Examples:
- ABO blood groups (A, B, AB, or O – nothing in between)
- Seed shape in peas (round or wrinkled)
- Seed colour in peas (green or yellow)
- Tongue rolling in humans (can roll or can’t roll)
Discontinuous variation = distinct options only
There are no values between – you either have the trait or you don’t.
Causes of Variation – Genes vs Environment
Discontinuous Variation:
- Caused by genes only
- Usually controlled by one or a few genes
- Environment does not affect the trait
🧾 Example:
- ABO blood group
- Seed shape in peas
- Tongue rolling
Continuous Variation:
- Caused by both genes and the environment
- Involves many genes (polygenic)
- Environmental factors (like diet, sunlight, exercise) influence the outcome
🧾 Example:
- Height in humans
- Body mass
- Skin colour
Discontinuous = genes only
Continuous = genes + environment
Investigating Continuous and Discontinuous Variation
What is Variation?
Variation means the differences between individuals of the same species. These differences can be:
- Continuous – a full range, no categories
- Discontinuous – distinct groups, no in-betweens
📊 Continuous Variation
🔍 Description:
- Traits show a range of values between two extremes
- No clear categories
- Usually caused by many genes and influenced by the environment
🧪 Examples to Investigate:
| Characteristic | How to Investigate |
|---|---|
| Height in humans | Measure using meter scale and plot on a graph to see a range |
| Body mass | Use weighing scale; shows wide variation |
| Leaf length in plants | Measure from the same species grown in different areas |
| Skin colour | Observe and record tones – gradual range |
🧬 Discontinuous Variation
🔍 Description:
- Traits have set categories (either/or)
- Caused by genes only
- No blending or middle values
🧪 Examples to Investigate:
| Characteristic | How to Investigate |
|---|---|
| ABO blood group | Blood test shows A, B, AB, or O only |
| Seed shape in peas | Observe round or wrinkled seeds |
| Tongue rolling | Survey: can roll or can’t roll |
| Earlobe attachment | Check if earlobes are free or attached |
🧠 Summary Table:
| Type of Variation | Range or Categories | Caused By | Examples |
|---|---|---|---|
| Continuous | Range | Genes + Environment | Height, body mass, leaf length |
| Discontinuous | Fixed categories | Genes only | Blood group, tongue rolling |
Mutation – A Genetic Change
What is a Mutation?
A mutation is a change in the DNA – the genetic material of a cell.
It is a genetic change that can alter the way a gene works or how a trait is expressed.
🔍 Key Points:
- Mutations can happen spontaneously (by chance)
- They may also be caused by external factors like radiation or certain chemicals
- Mutations may be harmful, beneficial, or have no effect at all
🧾 Example:
- A mutation in a gene that controls skin pigment might lead to albinism
- A mutation in a cancer-related gene might cause uncontrolled cell growth
Mutation = a change in the genetic code, which may affect how traits are passed on or expressed
Mutation and the Formation of New Alleles
Statement:
Mutation is the process by which new alleles are formed.
🔍 Explanation:
A mutation is a change in the DNA sequence of a gene.
When this change occurs, it can create a new version of the gene – called an allele.
This means that mutations introduce genetic variety into a population by producing new alleles that were not present before.
🧾 Example:
- A mutation in a gene controlling flower colour might create a new petal colour that didn’t exist earlier
- This new colour is due to a new allele formed by mutation
Without mutation, all alleles would stay the same, and evolution and natural selection wouldn’t happen.
Factors That Increase Mutation Rate
Statement:
Ionising radiation and some chemicals increase the rate of mutation.
🔍 Explanation:
While mutations can happen naturally, some factors make them more likely to occur. These are called mutagens.
🧪 Key Mutagens:
| Type | Examples | Effect |
|---|---|---|
| Ionising Radiation | X-rays, Gamma rays, UV light | Can damage DNA and cause mutations during cell division |
| Chemical Mutagens | Tobacco smoke, pesticides, industrial pollutants | Interfere with DNA structure or replication |
Exposure to high levels of radiation or harmful chemicals increases the risk of mutations, which may lead to genetic disorders or diseases like cancer.
Gene Mutation – A Change in the DNA Code
Definition:
A gene mutation is a random change in the base sequence of DNA.
🔍 What Does That Mean?
DNA is made up of a sequence of bases (A, T, C, G) that carry genetic instructions.
A mutation changes this sequence – even a small change can affect how a gene works.
🧾 Example:
Original DNA: A–T–G–C–C–T
Mutation changes it to: A–T–A–C–C–T
- Alter the protein made by the gene
- Have no effect
- Lead to a genetic disorder or trait change
Gene mutations are random and may have positive, negative, or no effects – depending on where and how the base sequence is changed.
Sources of Genetic Variation in Populations
Statement:
Mutation, meiosis, random mating, and random fertilisation are all important sources of genetic variation within a population.
🔍 How Each One Contributes:
| Source | Contribution to Genetic Variation |
|---|---|
| Mutation | A permanent change in DNA that can create new alleles and add new traits to the gene pool |
| Meiosis | Crossing over and independent assortment mix genetic material during gamete formation |
| Random Mating | Individuals choose mates freely, increasing genetic mixing across the population |
| Random Fertilisation | Any sperm can fuse with any egg – resulting in millions of unique offspring combinations |
These processes ensure that individuals in a population are genetically different, which is essential for natural selection and evolution.