Source: https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/16-2-5-chi-squared-test

[Revision notes](https://senecalearning.com/en-gb/revision-notes.md) › [International A Level](https://senecalearning.com/en-gb/revision-notes/international-a-level.md) › [Biology](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology.md) › [Cambridge International](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international.md) › Chi-Squared Test

# Chi-Squared Test

Biology: Cambridge International A Level, topic 16.2.5

## Overview of Chi-Squared

The chi-squared test is used in genetics to compare the goodness of fit of observed data with expected data. It tests if the difference between observed and expected values is due to chance.

### Inheritance

![Inheritance](https://image-v2.cdn.app.senecalearning.com/courseImages/biology/6.1.3%20-%20Genetic%20inheritance/6.1.6a%20Punnet%20square,%20pea%20plant%20colour-min.png)

- Genetic diagrams are used to predict the expected phenotypic ratio of offspring.
- Predictions are rarely 100% accurate because of the random nature of gametes fusing during fertilisation.
- Chi-squared is used to compare observed phenotypic ratios with expected ratios.
- Chi-squared tells us if the difference between the observed and expected ratios are due to chance.

### Requirements

![Requirements](https://image-v2.cdn.app.senecalearning.com/2018-09/19bdf08e-4df4-456b-96bf-03f08ce6f278/discrete.png)

- The Chi-squared test is used when:
 - Variation is discrete not continuous. This means the data are in categories (e.g. Aa and aa).
 - Data show absolute numbers (whole numbers), normally frequencies.

### Null hypothesis

![Null hypothesis](https://image-v2.cdn.app.senecalearning.com/2018-05/d9abeb87-aa7a-42d6-adef-7e16a3f6dc59/tick-305245_960_720.png)

- Before using chi-squared, a null hypothesis is stated.
- The null hypothesis is:
 - 'There is no significant difference between observed and expected data, the difference is due to chance'.
- The chi-squared test is used to reject or accept the null hypothesis.

### Equation

![Equation](https://image-v2.cdn.app.senecalearning.com/courseImages/chemistry/6.1.1%20Calculating%20rates%20of%20reactions/calculator-385506_640-min.jpg)

- The equation for chi-squared is:
 - $$\chi = \Sigma \; \frac{(O - E)^2}{E}$$
    - O = observed values.
    - E = expected values.

## Chi-Squared Test

The steps involved in applying the chi-squared test are:

### Equation

![Equation](https://image-v2.cdn.app.senecalearning.com/courseImages/chemistry/6.1.1%20Calculating%20rates%20of%20reactions/calculator-385506_640-min.jpg)

- The equation for chi-squared is:
- $$\chi = \Sigma \; \frac{O - E)^2}{E}$$
    - O = observed values.
    - E = expected values.

### 1) Calculate expected values

![1) Calculate expected values](https://image-v2.cdn.app.senecalearning.com/courseImages/biology/6.2.1%20-%20Variation%20and%20evolution/6.1.6a%20Punnet%20square,%20pea%20plant%20colour-min.jpg)

- To use the chi-squared equation, the expected values need to be calculated.
- Expected values are predicted using genetic diagrams.
- The expected values are the phenotypic ratios given by the genetic diagram.
- Compare the expected values with observed values and use these numbers in the equation.

### 2) Calculate chi-squared

![2) Calculate chi-squared](https://image-v2.cdn.app.senecalearning.com/courseImages/chemistry/6.1.1%20Calculating%20rates%20of%20reactions/calculator-385506_640-min.jpg)

- Using the chi-squared equation, calculate the chi-squared value.

### 3) Find the critical value

![3) Find the critical value](https://image-v2.cdn.app.senecalearning.com/courseImages/chemistry/6.1.1%20Calculating%20rates%20of%20reactions/calculator-385506_640-min.jpg)

- Calculate the degrees of freedom.
 - Degrees of freedom = the number of categories (e.g. phenotypes) − 1.
- Find the critical value that corresponds to the degrees of freedom in a probability distribution table at 0.05 significance level.

### 4) Accept the null hypothesis?

![4) Accept the null hypothesis?](https://image-v2.cdn.app.senecalearning.com/2018-05/d9abeb87-aa7a-42d6-adef-7e16a3f6dc59/tick-305245_960_720.png)

- Compare the chi-squared value to the critical value.
- If the chi-squared value is lower than the critical value - accept the null hypothesis.
 - The difference between observed and expected data is due to chance.

### Reject the null hypothesis?

![Reject the null hypothesis?](https://image-v2.cdn.app.senecalearning.com/2018-07/bbe834da-168d-4574-9f16-fde1649a1cca/Letter-X-Not-Allowed-No-Entry-.jpg)

- If the chi-squared value is greater than the critical value - reject the null hypothesis.
 - The difference between observed and expected data is NOT due to chance. 
 - This means we would get this chi-squared value in less than 5% of cases, which is very unlikely.

## Test yourself on Chi-Squared Test

- What are the requirements for the chi-squared test?
- Why are predictions of phenotypic ratios rarely 100% accurate?
- Which of the following statements about chi-squared are true?
- Stages of the Chi-Squared Test
- When is the null hypothesis rejected?

[Practise this topic for free](https://app.senecalearning.com/classroom/course/8d1a05c1-df92-4b13-9a69-b60acc7af79a/section/2bdb858b-59fa-4e6d-99a7-3c23d389b10f/session)

- Previous topic: [16.2.4 Multiple Alleles](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/16-2-4-multiple-alleles.md)
- Next topic: [16.2.6 Mutations](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/16-2-6-mutations.md)

## Biology: Cambridge International A Level contents

### 1 Cell Structure
- 1.1 Cell Structure
  - [1.1.1 Studying Cells - Microscopes](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/1-1-1-studying-cells-microscopes.md)
  - [1.1.2 Introduction to Eukaryotic & Prokaryotic Cells](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/1-1-2-introduction-to-eukaryotic-and-prokaryotic-cells.md)
  - [1.1.3 Ultrastructure of Eukaryotic Cells](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/1-1-3-ultrastructure-of-eukaryotic-cells.md)
  - [1.1.4 Ultrastructure of Eukaryotic Cells 2](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/1-1-4-ultrastructure-of-eukaryotic-cells-2.md)
  - [1.1.5 Ultrastructure of Eukaryotic Cells 3](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/1-1-5-ultrastructure-of-eukaryotic-cells-3.md)
  - [1.1.6 Prokaryotic Cells](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/1-1-6-prokaryotic-cells.md)
  - [1.1.7 Viruses](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/1-1-7-viruses.md)
  - [1.1.8 End of Topic Test - Cell Structure](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/1-1-8-end-of-topic-test-cell-structure.md)
  - [1.1.9 Exam-Style Question - Microscopes](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/1-1-9-exam-style-question-microscopes.md)
  - [1.1.10 A-A* (AO2/3) - Cell Structure](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/1-1-10-a-a-ao23-cell-structure.md)

### 2 Biological Molecules
- 2.1 Testing for Biological Modules
  - [2.1.1 Tests for Sugars & Starch](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/2-1-1-tests-for-sugars-and-starch.md)
  - [2.1.2 Tests for Lipids & Proteins](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/2-1-2-tests-for-lipids-and-proteins.md)
- 2.2 Carbohydrates & Lipids
  - [2.2.1 Monomers & Polymers](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/2-2-1-monomers-and-polymers.md)
  - [2.2.2 Carbohydrates](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/2-2-2-carbohydrates.md)
  - [2.2.3 Examples of Carbohydrates](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/2-2-3-examples-of-carbohydrates.md)
  - [2.2.4 Triglycerides & Phospholipids](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/2-2-4-triglycerides-and-phospholipids.md)
  - [2.2.5 Types of Fatty Acids](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/2-2-5-types-of-fatty-acids.md)
  - [2.2.6 Exam-Style Question - Carbohydrates](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/2-2-6-exam-style-question-carbohydrates.md)
  - [2.2.7 End of Topic Test - Carbohydrates & Lipids](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/2-2-7-end-of-topic-test-carbohydrates-and-lipids.md)
- 2.3 Proteins
  - [2.3.1 Amino Acids & Overview of Protein Structure](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/2-3-1-amino-acids-and-overview-of-protein-structure.md)
  - [2.3.2 Primary & Secondary Structure](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/2-3-2-primary-and-secondary-structure.md)
  - [2.3.3 Tertiary & Quaternary Structure](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/2-3-3-tertiary-and-quaternary-structure.md)
  - [2.3.4 Globular & Fibrous Proteins](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/2-3-4-globular-and-fibrous-proteins.md)
  - [2.3.5 Haemoglobin](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/2-3-5-haemoglobin.md)
  - [2.3.6 Collagen](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/2-3-6-collagen.md)
  - [2.3.7 End of Topic Test  - Proteins](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/2-3-7-end-of-topic-test-proteins.md)
- 2.4 Water
  - [2.4.1 Water Structure & Function](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/2-4-1-water-structure-and-function.md)
  - [2.4.2 End of Topic Test - Water](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/2-4-2-end-of-topic-test-water.md)

### 3 Enzymes
- 3.1 Enzymes
  - [3.1.1 Enzyme Action](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/3-1-1-enzyme-action.md)
  - [3.1.2 Factors Affecting Enzyme Activity](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/3-1-2-factors-affecting-enzyme-activity.md)
  - [3.1.3 Factors Affecting Enzyme Activity 2](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/3-1-3-factors-affecting-enzyme-activity-2.md)
  - [3.1.4 A-A* (AO2/3) - Enzymes](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/3-1-4-a-a-ao23-enzymes.md)

### 4 Cell Membranes & Transport
- 4.1 Biological Membranes
  - [4.1.1 Fluid Mosaic Model](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/4-1-1-fluid-mosaic-model.md)
  - [4.1.2 Cell Signalling](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/4-1-2-cell-signalling.md)
  - [4.1.3 End of Topic Test - Biological Membranes](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/4-1-3-end-of-topic-test-biological-membranes.md)
  - [4.1.4 Exam-Style Question - Membranes](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/4-1-4-exam-style-question-membranes.md)
- 4.2 Movement Into & Out of Cells
  - [4.2.1 Passive Movement Across the Membrane](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/4-2-1-passive-movement-across-the-membrane.md)
  - [4.2.2 Active Movement Across the Membrane](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/4-2-2-active-movement-across-the-membrane.md)
  - [4.2.3 Osmosis](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/4-2-3-osmosis.md)
  - [4.2.4 Osmosis & Plant Cells](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/4-2-4-osmosis-and-plant-cells.md)
  - [4.2.5 Surface Area to Volume Ratio](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/4-2-5-surface-area-to-volume-ratio.md)
  - [4.2.6 Adaptions to Surface Area to Volume Ratio](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/4-2-6-adaptions-to-surface-area-to-volume-ratio.md)
  - [4.2.7 Calculations - Volume](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/4-2-7-calculations-volume.md)
  - [4.2.8 Calculations - Surface Area](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/4-2-8-calculations-surface-area.md)

### 5 The Mitotic Cell Cycle
- 5.1 Replication & Division of Nuclei
  - [5.1.1 Chromosomes](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/5-1-1-chromosomes.md)
  - [5.1.2 The Cell Cycle](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/5-1-2-the-cell-cycle.md)
  - [5.1.3 Mitosis](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/5-1-3-mitosis.md)
  - [5.1.4 Role of Mitosis](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/5-1-4-role-of-mitosis.md)
  - [5.1.5 Stem Cells](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/5-1-5-stem-cells.md)
  - [5.1.6 Mitosis & Cancer](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/5-1-6-mitosis-and-cancer.md)
  - [5.1.7 End of Topic Test - Replication & Division](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/5-1-7-end-of-topic-test-replication-and-division.md)
  - [5.1.8 A-A* (AO2/3) - Cell Cycle & Transport](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/5-1-8-a-a-ao23-cell-cycle-and-transport.md)

### 6 Nucleic Acids & Protein Synthesis
- 6.1 Nucleic Acids
  - [6.1.1 Introduction to DNA & RNA](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/6-1-1-introduction-to-dna-and-rna.md)
  - [6.1.2 Nucleotides](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/6-1-2-nucleotides.md)
  - [6.1.3 Polynucleotides](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/6-1-3-polynucleotides.md)
  - [6.1.4 ADP & ATP](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/6-1-4-adp-and-atp.md)
  - [6.1.5 DNA](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/6-1-5-dna.md)
  - [6.1.6 DNA Replication](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/6-1-6-dna-replication.md)
  - [6.1.7 RNA](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/6-1-7-rna.md)
  - [6.1.8 End of Topic Test - Nucleic Acids & ATP](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/6-1-8-end-of-topic-test-nucleic-acids-and-atp.md)
- 6.2 Protein Synthesis
  - [6.2.1 The Genetic Code](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/6-2-1-the-genetic-code.md)
  - [6.2.2 Polypeptide Synthesis](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/6-2-2-polypeptide-synthesis.md)
  - [6.2.3 RNA Processing in Eukaryotes](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/6-2-3-rna-processing-in-eukaryotes.md)
  - [6.2.4 Mutations](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/6-2-4-mutations.md)
  - [6.2.5 Exam-Style Question - Protein Synthesis](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/6-2-5-exam-style-question-protein-synthesis.md)
  - [6.2.6 A-A* (AO2/3) - Coronavirus Translation](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/6-2-6-a-a-ao23-coronavirus-translation.md)

### 7 Transport in Plants
- 7.1 Transport in Plants
  - [7.1.1 Plant Vessels](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/7-1-1-plant-vessels.md)
  - [7.1.2 Xylem](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/7-1-2-xylem.md)
  - [7.1.3 Investigating Transpiration](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/7-1-3-investigating-transpiration.md)
  - [7.1.4 Adaptations to Water Availability](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/7-1-4-adaptations-to-water-availability.md)
  - [7.1.5 Phloem](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/7-1-5-phloem.md)
  - [7.1.6 End of Topic Test - Transport in Animals & Plants](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/7-1-6-end-of-topic-test-transport-in-animals-and-plants.md)
  - [7.1.7 A-A* (AO2/3) - Transport in Animals & Plants](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/7-1-7-a-a-ao23-transport-in-animals-and-plants.md)

### 8 Transport in Mammals
- 8.1 Circulatory System
  - [8.1.1 Double Circulation](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/8-1-1-double-circulation.md)
  - [8.1.2 Blood Vessels](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/8-1-2-blood-vessels.md)
  - [8.1.3 Important Blood Vessels](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/8-1-3-important-blood-vessels.md)
  - [8.1.4 Blood Cells](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/8-1-4-blood-cells.md)
  - [8.1.5 Tissue Fluid](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/8-1-5-tissue-fluid.md)
- 8.2 Transport of Oxygen & Carbon Dioxide
  - [8.2.1 Haemoglobin](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/8-2-1-haemoglobin.md)
  - [8.2.2 Oxygen Dissociation](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/8-2-2-oxygen-dissociation.md)
- 8.3 The Heart
  - [8.3.1 Structure of the Heart](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/8-3-1-structure-of-the-heart.md)
  - [8.3.2 The Cardiac Cycle](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/8-3-2-the-cardiac-cycle.md)
  - [8.3.3 Coordination of Heart Action](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/8-3-3-coordination-of-heart-action.md)

### 9 Gas Exchange
- 9.1 Gas Exchange System
  - [9.1.1 Mammalian Gas Exchange](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/9-1-1-mammalian-gas-exchange.md)
  - [9.1.2 Smoking](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/9-1-2-smoking.md)

### 10 Infectious Diseases
- 10.1 Infectious Diseases
  - [10.1.1 Transmission of Disease](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/10-1-1-transmission-of-disease.md)
  - [10.1.2 Tuberculosis](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/10-1-2-tuberculosis.md)
  - [10.1.3 HIV](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/10-1-3-hiv.md)
  - [10.1.4 Malaria](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/10-1-4-malaria.md)
  - [10.1.5 Cholera](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/10-1-5-cholera.md)
  - [10.1.6 Measles](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/10-1-6-measles.md)
- 10.2 Antibiotics
  - [10.2.1 Antibiotics](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/10-2-1-antibiotics.md)

### 11 Immunity
- 11.1 The Immune System
  - [11.1.1 Phagocytes](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/11-1-1-phagocytes.md)
  - [11.1.2 Antigens](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/11-1-2-antigens.md)
  - [11.1.3 Lymphocytes](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/11-1-3-lymphocytes.md)
  - [11.1.4 Primary & Secondary Response](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/11-1-4-primary-and-secondary-response.md)
- 11.2 Antibodies & Vaccination
  - [11.2.1 Antibodies](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/11-2-1-antibodies.md)
  - [11.2.2 Monoclonal Antibodies](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/11-2-2-monoclonal-antibodies.md)
  - [11.2.3 Immunity](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/11-2-3-immunity.md)
  - [11.2.4 Vaccines](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/11-2-4-vaccines.md)
  - [11.2.5 Exam-Style Question - Immune System](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/11-2-5-exam-style-question-immune-system.md)
  - [11.2.6 A-A* (AO2/3) - Disease & Immune System](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/11-2-6-a-a-ao23-disease-and-immune-system.md)

### 12 Energy & Respiration (A2 Only)
- 12.1 Energy
  - [12.1.1 Energy & ATP](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/12-1-1-energy-and-atp.md)
  - [12.1.2 Respiratory Substrates & Quotient](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/12-1-2-respiratory-substrates-and-quotient.md)
  - [12.1.3 Respirometers](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/12-1-3-respirometers.md)
- 12.2 Respiration
  - [12.2.1 Introduction to Respiration](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/12-2-1-introduction-to-respiration.md)
  - [12.2.2 Anaerobic Respiration](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/12-2-2-anaerobic-respiration.md)
  - [12.2.3 Aerobic Respiration](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/12-2-3-aerobic-respiration.md)
  - [12.2.4 Chemiosmosis & Coenzymes](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/12-2-4-chemiosmosis-and-coenzymes.md)
  - [12.2.5 Mitochondria](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/12-2-5-mitochondria.md)
  - [12.2.6 Respiration Experiments](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/12-2-6-respiration-experiments.md)
  - [12.2.7 End of Topic Test - Respiration](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/12-2-7-end-of-topic-test-respiration.md)
  - [12.2.8 A-A* (AO3/4) - Respiration](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/12-2-8-a-a-ao34-respiration.md)

### 13 Photosynthesis (A2 Only)
- 13.1 Photosynthesis
  - [13.1.1 Introduction to Photosynthesis](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/13-1-1-introduction-to-photosynthesis.md)
  - [13.1.2 Light-Dependent Reactions](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/13-1-2-light-dependent-reactions.md)
  - [13.1.3 Light-Independent Reactions](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/13-1-3-light-independent-reactions.md)
  - [13.1.4 Limiting Factors](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/13-1-4-limiting-factors.md)
  - [13.1.5 Investigating Photosynthesis](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/13-1-5-investigating-photosynthesis.md)
  - [13.1.6 End of Topic Test - Photosynthesis](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/13-1-6-end-of-topic-test-photosynthesis.md)
  - [13.1.7 A-A* (AO3/4) - Photosynthesis](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/13-1-7-a-a-ao34-photosynthesis.md)

### 14 Homeostasis (A2 Only)
- 14.1 Homeostasis
  - [14.1.1 Homeostasis](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/14-1-1-homeostasis.md)
  - [14.1.2 Negative Feedback](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/14-1-2-negative-feedback.md)
- 14.2 The Kidney
  - [14.2.1 Kidneys & Osmoregulation](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/14-2-1-kidneys-and-osmoregulation.md)
- 14.3 Cell Signalling
  - [14.3.1 Messengers & cAMP](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/14-3-1-messengers-and-camp.md)
- 14.4 Blood Glucose Concentration
  - [14.4.1 Blood Glucose Concentration](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/14-4-1-blood-glucose-concentration.md)
  - [14.4.2 Controlling Blood Glucose](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/14-4-2-controlling-blood-glucose.md)
  - [14.4.3 Urinalysis](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/14-4-3-urinalysis.md)
- 14.5 Homeostasis in Plants
  - [14.5.1 Stomata & Guard Cells](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/14-5-1-stomata-and-guard-cells.md)

### 15 Control & Coordination (A2 Only)
- 15.1 Control & Coordination in Mammals
  - [15.1.1 Neurones](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/15-1-1-neurones.md)
  - [15.1.2 Receptors](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/15-1-2-receptors.md)
  - [15.1.3 Taste](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/15-1-3-taste.md)
  - [15.1.4 Reflexes](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/15-1-4-reflexes.md)
  - [15.1.5 Action Potentials](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/15-1-5-action-potentials.md)
  - [15.1.6 Saltatory Conduction](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/15-1-6-saltatory-conduction.md)
  - [15.1.7 Synapses](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/15-1-7-synapses.md)
  - [15.1.8 Cholinergic Synnapses](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/15-1-8-cholinergic-synnapses.md)
  - [15.1.9 Neuromuscular Junction](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/15-1-9-neuromuscular-junction.md)
  - [15.1.10 Skeletal Muscle](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/15-1-10-skeletal-muscle.md)
  - [15.1.11 Sliding Filament Theory Contraction](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/15-1-11-sliding-filament-theory-contraction.md)
  - [15.1.12 Sliding Filament Theory Contraction 2](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/15-1-12-sliding-filament-theory-contraction-2.md)
  - [15.1.13 Menstruation](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/15-1-13-menstruation.md)
  - [15.1.14 Contraceptive Pill](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/15-1-14-contraceptive-pill.md)
- 15.2 Control & Co-Ordination in Plants
  - [15.2.1 Venus Flytrap](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/15-2-1-venus-flytrap.md)
  - [15.2.2 Auixns](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/15-2-2-auixns.md)
  - [15.2.3 Gibberellins](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/15-2-3-gibberellins.md)

### 16 Inherited Change (A2 Only)
- 16.1 Passage of Information to Offspring
  - [16.1.1 Meiosis](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/16-1-1-meiosis.md)
  - [16.1.2 Gametogenesis](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/16-1-2-gametogenesis.md)
  - [16.1.3 Meiosis & Genetic Diversity](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/16-1-3-meiosis-and-genetic-diversity.md)
- 16.2 Genes & Phenotype
  - [16.2.1 Key Terms in Genetics](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/16-2-1-key-terms-in-genetics.md)
  - [16.2.2 Inheritance](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/16-2-2-inheritance.md)
  - [16.2.3 Linkage](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/16-2-3-linkage.md)
  - [16.2.4 Multiple Alleles](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/16-2-4-multiple-alleles.md)
  - [16.2.5 Chi-Squared Test](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/16-2-5-chi-squared-test.md)
  - [16.2.6 Mutations](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/16-2-6-mutations.md)
  - [16.2.7 Specific Mutations](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/16-2-7-specific-mutations.md)
  - [16.2.8 End of Topic Test - Genetics](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/16-2-8-end-of-topic-test-genetics.md)
  - [16.2.9 A-A* (AO2/3) - Genetics](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/16-2-9-a-a-ao23-genetics.md)
- 16.3 Gene Control
  - [16.3.1 lac Operon](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/16-3-1-lac-operon.md)
  - [16.3.2 Transcription Factors](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/16-3-2-transcription-factors.md)
  - [16.3.3 Gibberellins & DELLA Proteins](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/16-3-3-gibberellins-and-della-proteins.md)

### 17 Selection & Evolution (A2 Only)
- 17.1 Variation
  - [17.1.1 Phenotypic Variation](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/17-1-1-phenotypic-variation.md)
  - [17.1.2 Continuous & Discontinuous Variation](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/17-1-2-continuous-and-discontinuous-variation.md)
  - [17.1.3 t-Tests](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/17-1-3-t-tests.md)
- 17.2 Natural & Artificial Selection
  - [17.2.1 Natural Selection & Evolution](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/17-2-1-natural-selection-and-evolution.md)
  - [17.2.2 Types of Selection](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/17-2-2-types-of-selection.md)
  - [17.2.3 Types of Selection Summary](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/17-2-3-types-of-selection-summary.md)
  - [17.2.4 Genetic Drift & Founder Effect](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/17-2-4-genetic-drift-and-founder-effect.md)
  - [17.2.5 Populations](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/17-2-5-populations.md)
  - [17.2.6 Hardy-Weinberg Principle](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/17-2-6-hardy-weinberg-principle.md)
  - [17.2.7 Artificial Selection](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/17-2-7-artificial-selection.md)
- 17.3 Evolution
  - [17.3.1 Evolution by Natural Selection](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/17-3-1-evolution-by-natural-selection.md)
  - [17.3.2 Evidence for Evolution](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/17-3-2-evidence-for-evolution.md)
  - [17.3.3 Speciation](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/17-3-3-speciation.md)
  - [17.3.4 Extinction](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/17-3-4-extinction.md)

### 18 Classification & Conservation (A2 Only)
- 18.1 Biodiversity
  - [18.1.1 Key Terms](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/18-1-1-key-terms.md)
  - [18.1.2 Biodiversity](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/18-1-2-biodiversity.md)
  - [18.1.3 Sampling Methods](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/18-1-3-sampling-methods.md)
  - [18.1.4 Correlation](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/18-1-4-correlation.md)
  - [18.1.5 Spearman’s Rank](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/18-1-5-spearmans-rank.md)
  - [18.1.6 Measuring Biodiversity](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/18-1-6-measuring-biodiversity.md)
- 18.2 Classification
  - [18.2.1 3-Domain Classification System](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/18-2-1-3-domain-classification-system.md)
  - [18.2.2 Linnaean Classification System](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/18-2-2-linnaean-classification-system.md)
- 18.3 Conservation
  - [18.3.1 Importance of Biodiversity](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/18-3-1-importance-of-biodiversity.md)
  - [18.3.2 Conservation](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/18-3-2-conservation.md)
  - [18.3.3 Zoos, Seedbanks, & Other Conservation Methods](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/18-3-3-zoos-seedbanks-and-other-conservation-methods.md)
  - [18.3.4 Assisted Reproduction & Contraception](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/18-3-4-assisted-reproduction-and-contraception.md)
  - [18.3.5 Non-Governmental Organisations](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/18-3-5-non-governmental-organisations.md)

### 19 Genetic Technology (A2 Only)
- 19.1 Manipulating Genomes
  - [19.1.1 Recombinant DNA & Genetic Engineering](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/19-1-1-recombinant-dna-and-genetic-engineering.md)
  - [19.1.2 Restriction Endonucleases](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/19-1-2-restriction-endonucleases.md)
  - [19.1.3 Gel Electrophoresis](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/19-1-3-gel-electrophoresis.md)
  - [19.1.4 DNA Probes & Microarrays](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/19-1-4-dna-probes-and-microarrays.md)
  - [19.1.5 End of Topic Test - Manipulating Genomes](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/19-1-5-end-of-topic-test-manipulating-genomes.md)
  - [19.1.6 A-A* (AO2/3) - Manipulating Genomes](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/19-1-6-a-a-ao23-manipulating-genomes.md)
- 19.2 Genetic Technology Applied to Medicine
  - [19.2.1 Bioinformatics](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/19-2-1-bioinformatics.md)
  - [19.2.2 Applications of Recombinant DNA](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/19-2-2-applications-of-recombinant-dna.md)
  - [19.2.3 Genetic Screening](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/19-2-3-genetic-screening.md)
  - [19.2.4 Gene Therapy](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/19-2-4-gene-therapy.md)
  - [19.2.5 Ethics](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/19-2-5-ethics.md)
  - [19.2.6 DNA Profiling & Forensics](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/19-2-6-dna-profiling-and-forensics.md)
- 19.3 Genetically Modified Organisms in Agriculture
  - [19.3.1 GM Crops & Livestock](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/19-3-1-gm-crops-and-livestock.md)
  - [19.3.2 Ethics](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/19-3-2-ethics.md)