Source: https://senecalearning.com/en-gb/revision-notes/igcse/biology/edexcel-igcse/higher/4-3-1-alleles

[Revision notes](https://senecalearning.com/en-gb/revision-notes.md) › [IGCSE](https://senecalearning.com/en-gb/revision-notes/igcse.md) › [Biology](https://senecalearning.com/en-gb/revision-notes/igcse/biology.md) › [Edexcel IGCSE](https://senecalearning.com/en-gb/revision-notes/igcse/biology/edexcel-igcse.md) › [Higher](https://senecalearning.com/en-gb/revision-notes/igcse/biology/edexcel-igcse/higher.md) › Alleles

# Alleles

Biology: Edexcel IGCSE, topic 4.3.1

## Alleles

Alleles are different forms of the same gene. Humans have pairs of every gene and in one gene, each half of the pair may have different alleles. People's characteristics are determined by the alleles that they have. Alleles can either be dominant or recessive:

### Dominant Alleles

![Dominant Alleles](https://image-v2.cdn.app.senecalearning.com/courseImages/biology/6.1.3%20-%20Genetic%20inheritance/man-461195_640-min.jpg)

- A dominant allele is always expressed, regardless of the identity of the other allele. 
- It only needs one copy present to be expressed (BB or Bb).
- It is represented by a capital letter, e.g. B.
- If B is the allele for brown eyes: 
 - When a person has a copy of the B allele, they will have brown eyes, no matter what other allele is present.

### Recessive Alleles

![Recessive Alleles](https://image-v2.cdn.app.senecalearning.com/courseImages/biology/6.1.3%20-%20Genetic%20inheritance/mug-2825127_640-min.jpg)

- A recessive allele is only expressed if the other allele is also recessive. 
- It is represented by a lowercase letter e.g. b. It needs two copies to be present to be expressed (bb).
- If b is the allele for blue eyes:
 - A person can only have blue eyes if both of their alleles are b.

## Test yourself on Alleles

[Practise this topic for free](https://app.senecalearning.com/classroom/course/e1a7eee0-26dc-11e8-8fe0-51e4f27b102c/section/62b0d2a0-3d7a-11e8-970e-d52a528a52e5/session)

- Previous topic: [4.2.9 End of Topic Test - Reproduction & Genetics](https://senecalearning.com/en-gb/revision-notes/igcse/biology/edexcel-igcse/higher/4-2-9-end-of-topic-test-reproduction-and-genetics.md)
- Next topic: [4.3.2 Phenotype](https://senecalearning.com/en-gb/revision-notes/igcse/biology/edexcel-igcse/higher/4-3-2-phenotype.md)

## Biology: Edexcel IGCSE contents

### 1 The Nature & Variety of Living Organisms
- 1.1 Characteristics of Living Organisms
  - [1.1.1 MRS GRENC](https://senecalearning.com/en-gb/revision-notes/igcse/biology/edexcel-igcse/higher/1-1-1-mrs-grenc.md)
  - [1.1.2 M.R.S.](https://senecalearning.com/en-gb/revision-notes/igcse/biology/edexcel-igcse/higher/1-1-2-mrs.md)
  - [1.1.3 G.R.E.N.C.](https://senecalearning.com/en-gb/revision-notes/igcse/biology/edexcel-igcse/higher/1-1-3-grenc.md)
- 1.2 Variety of Living Organisms
  - [1.2.1 Types of Cells](https://senecalearning.com/en-gb/revision-notes/igcse/biology/edexcel-igcse/higher/1-2-1-types-of-cells.md)
  - [1.2.2 Properties of Prokaryotes](https://senecalearning.com/en-gb/revision-notes/igcse/biology/edexcel-igcse/higher/1-2-2-properties-of-prokaryotes.md)
  - [1.2.3 Structure of Eukaryotic Cells](https://senecalearning.com/en-gb/revision-notes/igcse/biology/edexcel-igcse/higher/1-2-3-structure-of-eukaryotic-cells.md)
  - [1.2.4 Organisms: Animals & Plants](https://senecalearning.com/en-gb/revision-notes/igcse/biology/edexcel-igcse/higher/1-2-4-organisms-animals-and-plants.md)
  - [1.2.5 Other Kingdoms](https://senecalearning.com/en-gb/revision-notes/igcse/biology/edexcel-igcse/higher/1-2-5-other-kingdoms.md)
  - [1.2.6 Pathogens](https://senecalearning.com/en-gb/revision-notes/igcse/biology/edexcel-igcse/higher/1-2-6-pathogens.md)
  - [1.2.7 Viruses](https://senecalearning.com/en-gb/revision-notes/igcse/biology/edexcel-igcse/higher/1-2-7-viruses.md)

### 2 Structure & Functions in Living Organisms
- 2.1 Organisation Structure
  - [2.1.1 Cells](https://senecalearning.com/en-gb/revision-notes/igcse/biology/edexcel-igcse/higher/2-1-1-cells.md)
  - [2.1.2 Tissues](https://senecalearning.com/en-gb/revision-notes/igcse/biology/edexcel-igcse/higher/2-1-2-tissues.md)
  - [2.1.3 Organs](https://senecalearning.com/en-gb/revision-notes/igcse/biology/edexcel-igcse/higher/2-1-3-organs.md)
  - [2.1.4 Organ Systems](https://senecalearning.com/en-gb/revision-notes/igcse/biology/edexcel-igcse/higher/2-1-4-organ-systems.md)
  - [2.1.5 Organisms](https://senecalearning.com/en-gb/revision-notes/igcse/biology/edexcel-igcse/higher/2-1-5-organisms.md)
  - [2.1.6 Animal cells](https://senecalearning.com/en-gb/revision-notes/igcse/biology/edexcel-igcse/higher/2-1-6-animal-cells.md)
  - [2.1.7 Plant cells](https://senecalearning.com/en-gb/revision-notes/igcse/biology/edexcel-igcse/higher/2-1-7-plant-cells.md)
  - [2.1.8 Cell Differentiation](https://senecalearning.com/en-gb/revision-notes/igcse/biology/edexcel-igcse/higher/2-1-8-cell-differentiation.md)
  - [2.1.9 Stem Cells](https://senecalearning.com/en-gb/revision-notes/igcse/biology/edexcel-igcse/higher/2-1-9-stem-cells.md)
  - [2.1.10 Uses of Stem Cells](https://senecalearning.com/en-gb/revision-notes/igcse/biology/edexcel-igcse/higher/2-1-10-uses-of-stem-cells.md)
  - [2.1.11 Disadvantages of Stem Cells](https://senecalearning.com/en-gb/revision-notes/igcse/biology/edexcel-igcse/higher/2-1-11-disadvantages-of-stem-cells.md)
  - [2.1.12 End of Topic Test - Cell Types & Structures](https://senecalearning.com/en-gb/revision-notes/igcse/biology/edexcel-igcse/higher/2-1-12-end-of-topic-test-cell-types-and-structures.md)
  - [2.1.13 Exam-Style Questions - Cells](https://senecalearning.com/en-gb/revision-notes/igcse/biology/edexcel-igcse/higher/2-1-13-exam-style-questions-cells.md)
  - [2.1.14 Diffusion](https://senecalearning.com/en-gb/revision-notes/igcse/biology/edexcel-igcse/higher/2-1-14-diffusion.md)
  - [2.1.15 Factors affecting Diffusion](https://senecalearning.com/en-gb/revision-notes/igcse/biology/edexcel-igcse/higher/2-1-15-factors-affecting-diffusion.md)
  - [2.1.16 Surface Area to Volume Ratio](https://senecalearning.com/en-gb/revision-notes/igcse/biology/edexcel-igcse/higher/2-1-16-surface-area-to-volume-ratio.md)
  - [2.1.17 Diffusion - Experiment](https://senecalearning.com/en-gb/revision-notes/igcse/biology/edexcel-igcse/higher/2-1-17-diffusion-experiment.md)
  - [2.1.18 Osmosis](https://senecalearning.com/en-gb/revision-notes/igcse/biology/edexcel-igcse/higher/2-1-18-osmosis.md)
  - [2.1.19 Required Practical - Osmosis](https://senecalearning.com/en-gb/revision-notes/igcse/biology/edexcel-igcse/higher/2-1-19-required-practical-osmosis.md)
  - [2.1.20 Active Transport](https://senecalearning.com/en-gb/revision-notes/igcse/biology/edexcel-igcse/higher/2-1-20-active-transport.md)
  - [2.1.21 Transport in Cells](https://senecalearning.com/en-gb/revision-notes/igcse/biology/edexcel-igcse/higher/2-1-21-transport-in-cells.md)
  - [2.1.22 End of Topic Test - Cell Transport](https://senecalearning.com/en-gb/revision-notes/igcse/biology/edexcel-igcse/higher/2-1-22-end-of-topic-test-cell-transport.md)
  - [2.1.23 Grade 9 - Cell Transport](https://senecalearning.com/en-gb/revision-notes/igcse/biology/edexcel-igcse/higher/2-1-23-grade-9-cell-transport.md)
  - [2.1.24 Exam-Style Questions - Substance Exchange](https://senecalearning.com/en-gb/revision-notes/igcse/biology/edexcel-igcse/higher/2-1-24-exam-style-questions-substance-exchange.md)
- 2.2 Biological Molecules
  - [2.2.1 Chemical Elements](https://senecalearning.com/en-gb/revision-notes/igcse/biology/edexcel-igcse/higher/2-2-1-chemical-elements.md)
  - [2.2.2 Larger Molecules](https://senecalearning.com/en-gb/revision-notes/igcse/biology/edexcel-igcse/higher/2-2-2-larger-molecules.md)
  - [2.2.3 Required Practical - Food Testing](https://senecalearning.com/en-gb/revision-notes/igcse/biology/edexcel-igcse/higher/2-2-3-required-practical-food-testing.md)
  - [2.2.4 Enzymes](https://senecalearning.com/en-gb/revision-notes/igcse/biology/edexcel-igcse/higher/2-2-4-enzymes.md)
  - [2.2.5 Rate of Enzyme Action](https://senecalearning.com/en-gb/revision-notes/igcse/biology/edexcel-igcse/higher/2-2-5-rate-of-enzyme-action.md)
  - [2.2.6 Investigating Effect of pH on Amylase](https://senecalearning.com/en-gb/revision-notes/igcse/biology/edexcel-igcse/higher/2-2-6-investigating-effect-of-ph-on-amylase.md)
  - [2.2.7 Grade 9 - Enzymes](https://senecalearning.com/en-gb/revision-notes/igcse/biology/edexcel-igcse/higher/2-2-7-grade-9-enzymes.md)
- 2.3 Nutrition
  - [2.3.1 Photosynthesis](https://senecalearning.com/en-gb/revision-notes/igcse/biology/edexcel-igcse/higher/2-3-1-photosynthesis.md)
  - [2.3.2 Photosynthesis Equation](https://senecalearning.com/en-gb/revision-notes/igcse/biology/edexcel-igcse/higher/2-3-2-photosynthesis-equation.md)
  - [2.3.3 Factors affecting Rate of Photosynthesis](https://senecalearning.com/en-gb/revision-notes/igcse/biology/edexcel-igcse/higher/2-3-3-factors-affecting-rate-of-photosynthesis.md)
  - [2.3.4 Photosynthesis - Calculations](https://senecalearning.com/en-gb/revision-notes/igcse/biology/edexcel-igcse/higher/2-3-4-photosynthesis-calculations.md)
  - [2.3.5 Investigating Photosynthesis](https://senecalearning.com/en-gb/revision-notes/igcse/biology/edexcel-igcse/higher/2-3-5-investigating-photosynthesis.md)
  - [2.3.6 Testing Leaves for Starch](https://senecalearning.com/en-gb/revision-notes/igcse/biology/edexcel-igcse/higher/2-3-6-testing-leaves-for-starch.md)
  - [2.3.7 Grade 9 - Photosynthesis Experiment](https://senecalearning.com/en-gb/revision-notes/igcse/biology/edexcel-igcse/higher/2-3-7-grade-9-photosynthesis-experiment.md)
  - [2.3.8 Exam-Style Questions - Rate of Photosynthesis](https://senecalearning.com/en-gb/revision-notes/igcse/biology/edexcel-igcse/higher/2-3-8-exam-style-questions-rate-of-photosynthesis.md)
  - [2.3.9 Leaf Adaptations](https://senecalearning.com/en-gb/revision-notes/igcse/biology/edexcel-igcse/higher/2-3-9-leaf-adaptations.md)
  - [2.3.10 Ion Deficiencies](https://senecalearning.com/en-gb/revision-notes/igcse/biology/edexcel-igcse/higher/2-3-10-ion-deficiencies.md)
  - [2.3.11 Human Diet](https://senecalearning.com/en-gb/revision-notes/igcse/biology/edexcel-igcse/higher/2-3-11-human-diet.md)
  - [2.3.12 Vitamins and Minerals](https://senecalearning.com/en-gb/revision-notes/igcse/biology/edexcel-igcse/higher/2-3-12-vitamins-and-minerals.md)
  - [2.3.13 Dietary Needs](https://senecalearning.com/en-gb/revision-notes/igcse/biology/edexcel-igcse/higher/2-3-13-dietary-needs.md)
  - [2.3.14 Human Digestive System](https://senecalearning.com/en-gb/revision-notes/igcse/biology/edexcel-igcse/higher/2-3-14-human-digestive-system.md)
  - [2.3.15 Peristalsis](https://senecalearning.com/en-gb/revision-notes/igcse/biology/edexcel-igcse/higher/2-3-15-peristalsis.md)
  - [2.3.16 Villi](https://senecalearning.com/en-gb/revision-notes/igcse/biology/edexcel-igcse/higher/2-3-16-villi.md)
  - [2.3.17 Digestive Enzymes](https://senecalearning.com/en-gb/revision-notes/igcse/biology/edexcel-igcse/higher/2-3-17-digestive-enzymes.md)
  - [2.3.18 Amylase](https://senecalearning.com/en-gb/revision-notes/igcse/biology/edexcel-igcse/higher/2-3-18-amylase.md)
  - [2.3.19 Protease](https://senecalearning.com/en-gb/revision-notes/igcse/biology/edexcel-igcse/higher/2-3-19-protease.md)
  - [2.3.20 Lipase](https://senecalearning.com/en-gb/revision-notes/igcse/biology/edexcel-igcse/higher/2-3-20-lipase.md)
  - [2.3.21 Bile](https://senecalearning.com/en-gb/revision-notes/igcse/biology/edexcel-igcse/higher/2-3-21-bile.md)
  - [2.3.22 End of Topic Test - Enzymes & Nutririon](https://senecalearning.com/en-gb/revision-notes/igcse/biology/edexcel-igcse/higher/2-3-22-end-of-topic-test-enzymes-and-nutririon.md)
  - [2.3.23 Exam-Style Questions - Enzymes & Digestion](https://senecalearning.com/en-gb/revision-notes/igcse/biology/edexcel-igcse/higher/2-3-23-exam-style-questions-enzymes-and-digestion.md)

### 3 Organism Functions
- 3.1 Respiration
  - [3.1.1 Respiration](https://senecalearning.com/en-gb/revision-notes/igcse/biology/edexcel-igcse/higher/3-1-1-respiration.md)
  - [3.1.2 Aerobic and Anaerobic](https://senecalearning.com/en-gb/revision-notes/igcse/biology/edexcel-igcse/higher/3-1-2-aerobic-and-anaerobic.md)
  - [3.1.3 Investigating Respiration](https://senecalearning.com/en-gb/revision-notes/igcse/biology/edexcel-igcse/higher/3-1-3-investigating-respiration.md)
  - [3.1.4 End of Topic Test - Respiration](https://senecalearning.com/en-gb/revision-notes/igcse/biology/edexcel-igcse/higher/3-1-4-end-of-topic-test-respiration.md)
- 3.2 Gas Exchange
  - [3.2.1 Guard Cells & Stomata](https://senecalearning.com/en-gb/revision-notes/igcse/biology/edexcel-igcse/higher/3-2-1-guard-cells-and-stomata.md)
  - [3.2.2 States of Stomata](https://senecalearning.com/en-gb/revision-notes/igcse/biology/edexcel-igcse/higher/3-2-2-states-of-stomata.md)
  - [3.2.3 Plant Gas Exchange](https://senecalearning.com/en-gb/revision-notes/igcse/biology/edexcel-igcse/higher/3-2-3-plant-gas-exchange.md)
  - [3.2.4 Human Thorax](https://senecalearning.com/en-gb/revision-notes/igcse/biology/edexcel-igcse/higher/3-2-4-human-thorax.md)
  - [3.2.5 Breathing Process](https://senecalearning.com/en-gb/revision-notes/igcse/biology/edexcel-igcse/higher/3-2-5-breathing-process.md)
  - [3.2.6 Gas Exchange in Lungs](https://senecalearning.com/en-gb/revision-notes/igcse/biology/edexcel-igcse/higher/3-2-6-gas-exchange-in-lungs.md)
  - [3.2.7 Exchange Surfaces](https://senecalearning.com/en-gb/revision-notes/igcse/biology/edexcel-igcse/higher/3-2-7-exchange-surfaces.md)
  - [3.2.8 Alveoli](https://senecalearning.com/en-gb/revision-notes/igcse/biology/edexcel-igcse/higher/3-2-8-alveoli.md)
  - [3.2.9 Breath Composition](https://senecalearning.com/en-gb/revision-notes/igcse/biology/edexcel-igcse/higher/3-2-9-breath-composition.md)
  - [3.2.10 Exercise and Breathing](https://senecalearning.com/en-gb/revision-notes/igcse/biology/edexcel-igcse/higher/3-2-10-exercise-and-breathing.md)
  - [3.2.11 Smoking](https://senecalearning.com/en-gb/revision-notes/igcse/biology/edexcel-igcse/higher/3-2-11-smoking.md)
- 3.3 Transport
  - [3.3.1 Surface Area to Volume Ratio](https://senecalearning.com/en-gb/revision-notes/igcse/biology/edexcel-igcse/higher/3-3-1-surface-area-to-volume-ratio.md)
  - [3.3.2 Exchange Surfaces](https://senecalearning.com/en-gb/revision-notes/igcse/biology/edexcel-igcse/higher/3-3-2-exchange-surfaces.md)
  - [3.3.3 Examples of Exchange Surfaces](https://senecalearning.com/en-gb/revision-notes/igcse/biology/edexcel-igcse/higher/3-3-3-examples-of-exchange-surfaces.md)
  - [3.3.4 Root Hair Cells](https://senecalearning.com/en-gb/revision-notes/igcse/biology/edexcel-igcse/higher/3-3-4-root-hair-cells.md)
  - [3.3.5 Transpiration](https://senecalearning.com/en-gb/revision-notes/igcse/biology/edexcel-igcse/higher/3-3-5-transpiration.md)
  - [3.3.6 Rate of Transpiration](https://senecalearning.com/en-gb/revision-notes/igcse/biology/edexcel-igcse/higher/3-3-6-rate-of-transpiration.md)
  - [3.3.7 Measuring Transpiration](https://senecalearning.com/en-gb/revision-notes/igcse/biology/edexcel-igcse/higher/3-3-7-measuring-transpiration.md)
  - [3.3.8 Xylem](https://senecalearning.com/en-gb/revision-notes/igcse/biology/edexcel-igcse/higher/3-3-8-xylem.md)
  - [3.3.9 Translocation](https://senecalearning.com/en-gb/revision-notes/igcse/biology/edexcel-igcse/higher/3-3-9-translocation.md)
  - [3.3.10 Blood Components](https://senecalearning.com/en-gb/revision-notes/igcse/biology/edexcel-igcse/higher/3-3-10-blood-components.md)
  - [3.3.11 Plasma](https://senecalearning.com/en-gb/revision-notes/igcse/biology/edexcel-igcse/higher/3-3-11-plasma.md)
  - [3.3.12 Red Blood Cell](https://senecalearning.com/en-gb/revision-notes/igcse/biology/edexcel-igcse/higher/3-3-12-red-blood-cell.md)
  - [3.3.13 Platelets](https://senecalearning.com/en-gb/revision-notes/igcse/biology/edexcel-igcse/higher/3-3-13-platelets.md)
  - [3.3.14 The Immune System](https://senecalearning.com/en-gb/revision-notes/igcse/biology/edexcel-igcse/higher/3-3-14-the-immune-system.md)
  - [3.3.15 Antibodies](https://senecalearning.com/en-gb/revision-notes/igcse/biology/edexcel-igcse/higher/3-3-15-antibodies.md)
  - [3.3.16 Vaccination](https://senecalearning.com/en-gb/revision-notes/igcse/biology/edexcel-igcse/higher/3-3-16-vaccination.md)
  - [3.3.17 Types of Blood Vessel](https://senecalearning.com/en-gb/revision-notes/igcse/biology/edexcel-igcse/higher/3-3-17-types-of-blood-vessel.md)
  - [3.3.18 Blood Vessels - Arteries](https://senecalearning.com/en-gb/revision-notes/igcse/biology/edexcel-igcse/higher/3-3-18-blood-vessels-arteries.md)
  - [3.3.19 Blood Vessels - Veins](https://senecalearning.com/en-gb/revision-notes/igcse/biology/edexcel-igcse/higher/3-3-19-blood-vessels-veins.md)
  - [3.3.20 Blood Vessels - Capillaries](https://senecalearning.com/en-gb/revision-notes/igcse/biology/edexcel-igcse/higher/3-3-20-blood-vessels-capillaries.md)
  - [3.3.21 Function of the Heart](https://senecalearning.com/en-gb/revision-notes/igcse/biology/edexcel-igcse/higher/3-3-21-function-of-the-heart.md)
  - [3.3.22 Structure of the Heart](https://senecalearning.com/en-gb/revision-notes/igcse/biology/edexcel-igcse/higher/3-3-22-structure-of-the-heart.md)
  - [3.3.23 Heart Rate](https://senecalearning.com/en-gb/revision-notes/igcse/biology/edexcel-igcse/higher/3-3-23-heart-rate.md)
  - [3.3.24 Heart Disease](https://senecalearning.com/en-gb/revision-notes/igcse/biology/edexcel-igcse/higher/3-3-24-heart-disease.md)
  - [3.3.25 Double Circulatory System](https://senecalearning.com/en-gb/revision-notes/igcse/biology/edexcel-igcse/higher/3-3-25-double-circulatory-system.md)
  - [3.3.26 Important Blood Vessels](https://senecalearning.com/en-gb/revision-notes/igcse/biology/edexcel-igcse/higher/3-3-26-important-blood-vessels.md)
  - [3.3.27 End of Topic - Transport](https://senecalearning.com/en-gb/revision-notes/igcse/biology/edexcel-igcse/higher/3-3-27-end-of-topic-transport.md)
  - [3.3.28 Exam-Style Questions - The Heart](https://senecalearning.com/en-gb/revision-notes/igcse/biology/edexcel-igcse/higher/3-3-28-exam-style-questions-the-heart.md)
- 3.4 Excretion
  - [3.4.1 Excretion](https://senecalearning.com/en-gb/revision-notes/igcse/biology/edexcel-igcse/higher/3-4-1-excretion.md)
  - [3.4.2 Kidneys](https://senecalearning.com/en-gb/revision-notes/igcse/biology/edexcel-igcse/higher/3-4-2-kidneys.md)
  - [3.4.3 Urine Production](https://senecalearning.com/en-gb/revision-notes/igcse/biology/edexcel-igcse/higher/3-4-3-urine-production.md)
  - [3.4.4 Urinary System](https://senecalearning.com/en-gb/revision-notes/igcse/biology/edexcel-igcse/higher/3-4-4-urinary-system.md)
  - [3.4.5 Nephron](https://senecalearning.com/en-gb/revision-notes/igcse/biology/edexcel-igcse/higher/3-4-5-nephron.md)
  - [3.4.6 Water in Urine](https://senecalearning.com/en-gb/revision-notes/igcse/biology/edexcel-igcse/higher/3-4-6-water-in-urine.md)
- 3.5 Coordination & Response
  - [3.5.1 Homeostasis](https://senecalearning.com/en-gb/revision-notes/igcse/biology/edexcel-igcse/higher/3-5-1-homeostasis.md)
  - [3.5.2 Regulating Internal Conditions](https://senecalearning.com/en-gb/revision-notes/igcse/biology/edexcel-igcse/higher/3-5-2-regulating-internal-conditions.md)
  - [3.5.3 Coordinated Responses](https://senecalearning.com/en-gb/revision-notes/igcse/biology/edexcel-igcse/higher/3-5-3-coordinated-responses.md)
  - [3.5.4 Types of Tropisms](https://senecalearning.com/en-gb/revision-notes/igcse/biology/edexcel-igcse/higher/3-5-4-types-of-tropisms.md)
  - [3.5.5 Auxin and Phototropism](https://senecalearning.com/en-gb/revision-notes/igcse/biology/edexcel-igcse/higher/3-5-5-auxin-and-phototropism.md)
  - [3.5.6 Nervous System](https://senecalearning.com/en-gb/revision-notes/igcse/biology/edexcel-igcse/higher/3-5-6-nervous-system.md)
  - [3.5.7 Synapses](https://senecalearning.com/en-gb/revision-notes/igcse/biology/edexcel-igcse/higher/3-5-7-synapses.md)
  - [3.5.8 Reflex Actions](https://senecalearning.com/en-gb/revision-notes/igcse/biology/edexcel-igcse/higher/3-5-8-reflex-actions.md)
  - [3.5.9 Reflex Arc](https://senecalearning.com/en-gb/revision-notes/igcse/biology/edexcel-igcse/higher/3-5-9-reflex-arc.md)
  - [3.5.10 Sense Organs](https://senecalearning.com/en-gb/revision-notes/igcse/biology/edexcel-igcse/higher/3-5-10-sense-organs.md)
  - [3.5.11 The Eye](https://senecalearning.com/en-gb/revision-notes/igcse/biology/edexcel-igcse/higher/3-5-11-the-eye.md)
  - [3.5.12 Eye - Focusing](https://senecalearning.com/en-gb/revision-notes/igcse/biology/edexcel-igcse/higher/3-5-12-eye-focusing.md)
  - [3.5.13 Eye - Adapting to Light](https://senecalearning.com/en-gb/revision-notes/igcse/biology/edexcel-igcse/higher/3-5-13-eye-adapting-to-light.md)
  - [3.5.14 Thermoregulation](https://senecalearning.com/en-gb/revision-notes/igcse/biology/edexcel-igcse/higher/3-5-14-thermoregulation.md)
  - [3.5.15 Sweating and Shivering](https://senecalearning.com/en-gb/revision-notes/igcse/biology/edexcel-igcse/higher/3-5-15-sweating-and-shivering.md)
  - [3.5.16 Thermoregulation - Blood Vessels](https://senecalearning.com/en-gb/revision-notes/igcse/biology/edexcel-igcse/higher/3-5-16-thermoregulation-blood-vessels.md)
  - [3.5.17 Exam-Style Questions - Homeostasis](https://senecalearning.com/en-gb/revision-notes/igcse/biology/edexcel-igcse/higher/3-5-17-exam-style-questions-homeostasis.md)
  - [3.5.18 Endocrine Glands](https://senecalearning.com/en-gb/revision-notes/igcse/biology/edexcel-igcse/higher/3-5-18-endocrine-glands.md)
  - [3.5.19 Hormones](https://senecalearning.com/en-gb/revision-notes/igcse/biology/edexcel-igcse/higher/3-5-19-hormones.md)
  - [3.5.20 Nervous vs Endocrine](https://senecalearning.com/en-gb/revision-notes/igcse/biology/edexcel-igcse/higher/3-5-20-nervous-vs-endocrine.md)
  - [3.5.21 Adrenaline](https://senecalearning.com/en-gb/revision-notes/igcse/biology/edexcel-igcse/higher/3-5-21-adrenaline.md)
  - [3.5.22 Glucose](https://senecalearning.com/en-gb/revision-notes/igcse/biology/edexcel-igcse/higher/3-5-22-glucose.md)
  - [3.5.23 Puberty](https://senecalearning.com/en-gb/revision-notes/igcse/biology/edexcel-igcse/higher/3-5-23-puberty.md)
  - [3.5.24 Menstruation](https://senecalearning.com/en-gb/revision-notes/igcse/biology/edexcel-igcse/higher/3-5-24-menstruation.md)
  - [3.5.25 Menstrual Cycle](https://senecalearning.com/en-gb/revision-notes/igcse/biology/edexcel-igcse/higher/3-5-25-menstrual-cycle.md)
  - [3.5.26 Menstrual Hormones](https://senecalearning.com/en-gb/revision-notes/igcse/biology/edexcel-igcse/higher/3-5-26-menstrual-hormones.md)
  - [3.5.27 Grade 9 - Hormonal Coordination](https://senecalearning.com/en-gb/revision-notes/igcse/biology/edexcel-igcse/higher/3-5-27-grade-9-hormonal-coordination.md)
  - [3.5.28 End of Topic Test - Coordination & Response](https://senecalearning.com/en-gb/revision-notes/igcse/biology/edexcel-igcse/higher/3-5-28-end-of-topic-test-coordination-and-response.md)

### 4 Reproduction & Inheritance
- 4.1 Reproduction
  - [4.1.1 Asexual Reproduction](https://senecalearning.com/en-gb/revision-notes/igcse/biology/edexcel-igcse/higher/4-1-1-asexual-reproduction.md)
  - [4.1.2 Sexual Reproduction](https://senecalearning.com/en-gb/revision-notes/igcse/biology/edexcel-igcse/higher/4-1-2-sexual-reproduction.md)
  - [4.1.3 Gamete vs Zygote](https://senecalearning.com/en-gb/revision-notes/igcse/biology/edexcel-igcse/higher/4-1-3-gamete-vs-zygote.md)
  - [4.1.4 Plant Reproduction](https://senecalearning.com/en-gb/revision-notes/igcse/biology/edexcel-igcse/higher/4-1-4-plant-reproduction.md)
  - [4.1.5 Flower Anatomy](https://senecalearning.com/en-gb/revision-notes/igcse/biology/edexcel-igcse/higher/4-1-5-flower-anatomy.md)
  - [4.1.6 Germination](https://senecalearning.com/en-gb/revision-notes/igcse/biology/edexcel-igcse/higher/4-1-6-germination.md)
  - [4.1.7 Pollination](https://senecalearning.com/en-gb/revision-notes/igcse/biology/edexcel-igcse/higher/4-1-7-pollination.md)
  - [4.1.8 Pollination Methods](https://senecalearning.com/en-gb/revision-notes/igcse/biology/edexcel-igcse/higher/4-1-8-pollination-methods.md)
  - [4.1.9 Male Reproductive System](https://senecalearning.com/en-gb/revision-notes/igcse/biology/edexcel-igcse/higher/4-1-9-male-reproductive-system.md)
  - [4.1.10 Female Reproductive System](https://senecalearning.com/en-gb/revision-notes/igcse/biology/edexcel-igcse/higher/4-1-10-female-reproductive-system.md)
  - [4.1.11 Pregnancy](https://senecalearning.com/en-gb/revision-notes/igcse/biology/edexcel-igcse/higher/4-1-11-pregnancy.md)
  - [4.1.12 Exam-Style Questions - Sexual Reproduction](https://senecalearning.com/en-gb/revision-notes/igcse/biology/edexcel-igcse/higher/4-1-12-exam-style-questions-sexual-reproduction.md)
- 4.2 Genetics
  - [4.2.1 Cell Division](https://senecalearning.com/en-gb/revision-notes/igcse/biology/edexcel-igcse/higher/4-2-1-cell-division.md)
  - [4.2.2 Meiosis](https://senecalearning.com/en-gb/revision-notes/igcse/biology/edexcel-igcse/higher/4-2-2-meiosis.md)
  - [4.2.3 Genome](https://senecalearning.com/en-gb/revision-notes/igcse/biology/edexcel-igcse/higher/4-2-3-genome.md)
  - [4.2.4 Genome Diagram](https://senecalearning.com/en-gb/revision-notes/igcse/biology/edexcel-igcse/higher/4-2-4-genome-diagram.md)
  - [4.2.5 DNA](https://senecalearning.com/en-gb/revision-notes/igcse/biology/edexcel-igcse/higher/4-2-5-dna.md)
  - [4.2.6 RNA](https://senecalearning.com/en-gb/revision-notes/igcse/biology/edexcel-igcse/higher/4-2-6-rna.md)
  - [4.2.7 Protein Synthesis](https://senecalearning.com/en-gb/revision-notes/igcse/biology/edexcel-igcse/higher/4-2-7-protein-synthesis.md)
  - [4.2.8 Synthesis Process](https://senecalearning.com/en-gb/revision-notes/igcse/biology/edexcel-igcse/higher/4-2-8-synthesis-process.md)
  - [4.2.9 End of Topic Test - Reproduction & Genetics](https://senecalearning.com/en-gb/revision-notes/igcse/biology/edexcel-igcse/higher/4-2-9-end-of-topic-test-reproduction-and-genetics.md)
- 4.3 Variation
  - [4.3.1 Alleles](https://senecalearning.com/en-gb/revision-notes/igcse/biology/edexcel-igcse/higher/4-3-1-alleles.md)
  - [4.3.2 Phenotype](https://senecalearning.com/en-gb/revision-notes/igcse/biology/edexcel-igcse/higher/4-3-2-phenotype.md)
  - [4.3.3 Monohybrid Inheritance](https://senecalearning.com/en-gb/revision-notes/igcse/biology/edexcel-igcse/higher/4-3-3-monohybrid-inheritance.md)
  - [4.3.4 Punnett Square](https://senecalearning.com/en-gb/revision-notes/igcse/biology/edexcel-igcse/higher/4-3-4-punnett-square.md)
  - [4.3.5 Sex Determination](https://senecalearning.com/en-gb/revision-notes/igcse/biology/edexcel-igcse/higher/4-3-5-sex-determination.md)
  - [4.3.6 Codominance](https://senecalearning.com/en-gb/revision-notes/igcse/biology/edexcel-igcse/higher/4-3-6-codominance.md)
  - [4.3.7 Cystic Fibrosis Punnett Square](https://senecalearning.com/en-gb/revision-notes/igcse/biology/edexcel-igcse/higher/4-3-7-cystic-fibrosis-punnett-square.md)
  - [4.3.8 Cystic Fibrosis Pedigree Diagram](https://senecalearning.com/en-gb/revision-notes/igcse/biology/edexcel-igcse/higher/4-3-8-cystic-fibrosis-pedigree-diagram.md)
  - [4.3.9 Variation](https://senecalearning.com/en-gb/revision-notes/igcse/biology/edexcel-igcse/higher/4-3-9-variation.md)
  - [4.3.10 Mutations](https://senecalearning.com/en-gb/revision-notes/igcse/biology/edexcel-igcse/higher/4-3-10-mutations.md)
  - [4.3.11 Natural Selection](https://senecalearning.com/en-gb/revision-notes/igcse/biology/edexcel-igcse/higher/4-3-11-natural-selection.md)
  - [4.3.12 Evolution](https://senecalearning.com/en-gb/revision-notes/igcse/biology/edexcel-igcse/higher/4-3-12-evolution.md)
  - [4.3.13 Resistant Bacteria](https://senecalearning.com/en-gb/revision-notes/igcse/biology/edexcel-igcse/higher/4-3-13-resistant-bacteria.md)
  - [4.3.14 End of Topic Test - Variation](https://senecalearning.com/en-gb/revision-notes/igcse/biology/edexcel-igcse/higher/4-3-14-end-of-topic-test-variation.md)
  - [4.3.15 Grade 9 - Evolution](https://senecalearning.com/en-gb/revision-notes/igcse/biology/edexcel-igcse/higher/4-3-15-grade-9-evolution.md)
  - [4.3.16 Exam-Style Questions - Cystic Fibrosis](https://senecalearning.com/en-gb/revision-notes/igcse/biology/edexcel-igcse/higher/4-3-16-exam-style-questions-cystic-fibrosis.md)

### 5 Ecology
- 5.1 Ecosystems
  - [5.1.1 Ecosystems](https://senecalearning.com/en-gb/revision-notes/igcse/biology/edexcel-igcse/higher/5-1-1-ecosystems.md)
  - [5.1.2 Resources](https://senecalearning.com/en-gb/revision-notes/igcse/biology/edexcel-igcse/higher/5-1-2-resources.md)
  - [5.1.3 Interdependence](https://senecalearning.com/en-gb/revision-notes/igcse/biology/edexcel-igcse/higher/5-1-3-interdependence.md)
  - [5.1.4 Growth Factors](https://senecalearning.com/en-gb/revision-notes/igcse/biology/edexcel-igcse/higher/5-1-4-growth-factors.md)
  - [5.1.5 Assessing Ecosystems](https://senecalearning.com/en-gb/revision-notes/igcse/biology/edexcel-igcse/higher/5-1-5-assessing-ecosystems.md)
  - [5.1.6 Quadrats](https://senecalearning.com/en-gb/revision-notes/igcse/biology/edexcel-igcse/higher/5-1-6-quadrats.md)
  - [5.1.7 Required Practical](https://senecalearning.com/en-gb/revision-notes/igcse/biology/edexcel-igcse/higher/5-1-7-required-practical.md)
  - [5.1.8 Exam-Style Questions - Sampling](https://senecalearning.com/en-gb/revision-notes/igcse/biology/edexcel-igcse/higher/5-1-8-exam-style-questions-sampling.md)
  - [5.1.9 Energy Flow](https://senecalearning.com/en-gb/revision-notes/igcse/biology/edexcel-igcse/higher/5-1-9-energy-flow.md)
  - [5.1.10 Food Chains](https://senecalearning.com/en-gb/revision-notes/igcse/biology/edexcel-igcse/higher/5-1-10-food-chains.md)
  - [5.1.11 Trophic Levels](https://senecalearning.com/en-gb/revision-notes/igcse/biology/edexcel-igcse/higher/5-1-11-trophic-levels.md)
  - [5.1.12 Decomposers](https://senecalearning.com/en-gb/revision-notes/igcse/biology/edexcel-igcse/higher/5-1-12-decomposers.md)
  - [5.1.13 Pyramids of Biomass](https://senecalearning.com/en-gb/revision-notes/igcse/biology/edexcel-igcse/higher/5-1-13-pyramids-of-biomass.md)
  - [5.1.14 Transfer of Biomass](https://senecalearning.com/en-gb/revision-notes/igcse/biology/edexcel-igcse/higher/5-1-14-transfer-of-biomass.md)
  - [5.1.15 Reasons for Wastage](https://senecalearning.com/en-gb/revision-notes/igcse/biology/edexcel-igcse/higher/5-1-15-reasons-for-wastage.md)
  - [5.1.16 End of Topic Test - Food and Trophic Levels](https://senecalearning.com/en-gb/revision-notes/igcse/biology/edexcel-igcse/higher/5-1-16-end-of-topic-test-food-and-trophic-levels.md)
  - [5.1.17 Recycling Materials](https://senecalearning.com/en-gb/revision-notes/igcse/biology/edexcel-igcse/higher/5-1-17-recycling-materials.md)
  - [5.1.18 Carbon Cycle](https://senecalearning.com/en-gb/revision-notes/igcse/biology/edexcel-igcse/higher/5-1-18-carbon-cycle.md)
  - [5.1.19 Exam-Style Questions - Carbon Cycle](https://senecalearning.com/en-gb/revision-notes/igcse/biology/edexcel-igcse/higher/5-1-19-exam-style-questions-carbon-cycle.md)
  - [5.1.20 Nitrogen Cycle](https://senecalearning.com/en-gb/revision-notes/igcse/biology/edexcel-igcse/higher/5-1-20-nitrogen-cycle.md)
  - [5.1.21 The Nitrogen Cycle and Microorganisms](https://senecalearning.com/en-gb/revision-notes/igcse/biology/edexcel-igcse/higher/5-1-21-the-nitrogen-cycle-and-microorganisms.md)
  - [5.1.22 End of Topic Test - Ecosystems](https://senecalearning.com/en-gb/revision-notes/igcse/biology/edexcel-igcse/higher/5-1-22-end-of-topic-test-ecosystems.md)
  - [5.1.23 Grade 9 - Ecosystems](https://senecalearning.com/en-gb/revision-notes/igcse/biology/edexcel-igcse/higher/5-1-23-grade-9-ecosystems.md)
- 5.2 Human Impacts
  - [5.2.1 Waste Management](https://senecalearning.com/en-gb/revision-notes/igcse/biology/edexcel-igcse/higher/5-2-1-waste-management.md)
  - [5.2.2 Eutrophication](https://senecalearning.com/en-gb/revision-notes/igcse/biology/edexcel-igcse/higher/5-2-2-eutrophication.md)
  - [5.2.3 Air Pollution](https://senecalearning.com/en-gb/revision-notes/igcse/biology/edexcel-igcse/higher/5-2-3-air-pollution.md)
  - [5.2.4 Deforestation](https://senecalearning.com/en-gb/revision-notes/igcse/biology/edexcel-igcse/higher/5-2-4-deforestation.md)
  - [5.2.5 The Greenhouse Effect](https://senecalearning.com/en-gb/revision-notes/igcse/biology/edexcel-igcse/higher/5-2-5-the-greenhouse-effect.md)
  - [5.2.6 Greenhouse Gases](https://senecalearning.com/en-gb/revision-notes/igcse/biology/edexcel-igcse/higher/5-2-6-greenhouse-gases.md)
  - [5.2.7 Global Warming](https://senecalearning.com/en-gb/revision-notes/igcse/biology/edexcel-igcse/higher/5-2-7-global-warming.md)
  - [5.2.8 End of Topic Test - Human Impacts](https://senecalearning.com/en-gb/revision-notes/igcse/biology/edexcel-igcse/higher/5-2-8-end-of-topic-test-human-impacts.md)
  - [5.2.9 Exam-Style Questions - Air Pollution](https://senecalearning.com/en-gb/revision-notes/igcse/biology/edexcel-igcse/higher/5-2-9-exam-style-questions-air-pollution.md)

### 6 Biological Resources
- 6.1 Food Production
  - [6.1.1 Intensive Farming  - Crop Yield](https://senecalearning.com/en-gb/revision-notes/igcse/biology/edexcel-igcse/higher/6-1-1-intensive-farming-crop-yield.md)
  - [6.1.2 Intensive Farming - Ion Deficiency](https://senecalearning.com/en-gb/revision-notes/igcse/biology/edexcel-igcse/higher/6-1-2-intensive-farming-ion-deficiency.md)
  - [6.1.3 Intensive Farming - Pest Control](https://senecalearning.com/en-gb/revision-notes/igcse/biology/edexcel-igcse/higher/6-1-3-intensive-farming-pest-control.md)
  - [6.1.4 Intensive Farming - Fish Farming](https://senecalearning.com/en-gb/revision-notes/igcse/biology/edexcel-igcse/higher/6-1-4-intensive-farming-fish-farming.md)
  - [6.1.5 Microorganisms in Food](https://senecalearning.com/en-gb/revision-notes/igcse/biology/edexcel-igcse/higher/6-1-5-microorganisms-in-food.md)
  - [6.1.6 Factors Affecting Microorganism Growth](https://senecalearning.com/en-gb/revision-notes/igcse/biology/edexcel-igcse/higher/6-1-6-factors-affecting-microorganism-growth.md)
  - [6.1.7 Industrial Fermentation](https://senecalearning.com/en-gb/revision-notes/igcse/biology/edexcel-igcse/higher/6-1-7-industrial-fermentation.md)
  - [6.1.8 Selective Breeding](https://senecalearning.com/en-gb/revision-notes/igcse/biology/edexcel-igcse/higher/6-1-8-selective-breeding.md)
  - [6.1.9 Selective Breeding Process](https://senecalearning.com/en-gb/revision-notes/igcse/biology/edexcel-igcse/higher/6-1-9-selective-breeding-process.md)
  - [6.1.10 Uses of Selective Breeding](https://senecalearning.com/en-gb/revision-notes/igcse/biology/edexcel-igcse/higher/6-1-10-uses-of-selective-breeding.md)
  - [6.1.11 Exam-Style Questions - Selective Breeding](https://senecalearning.com/en-gb/revision-notes/igcse/biology/edexcel-igcse/higher/6-1-11-exam-style-questions-selective-breeding.md)
- 6.2 Genetic Engineering
  - [6.2.1 Genetic Engineering](https://senecalearning.com/en-gb/revision-notes/igcse/biology/edexcel-igcse/higher/6-2-1-genetic-engineering.md)
  - [6.2.2 Uses of Genetic Engineering](https://senecalearning.com/en-gb/revision-notes/igcse/biology/edexcel-igcse/higher/6-2-2-uses-of-genetic-engineering.md)
  - [6.2.3 Producing Human Proteins in Bacteria](https://senecalearning.com/en-gb/revision-notes/igcse/biology/edexcel-igcse/higher/6-2-3-producing-human-proteins-in-bacteria.md)
  - [6.2.4 Tissue Culture](https://senecalearning.com/en-gb/revision-notes/igcse/biology/edexcel-igcse/higher/6-2-4-tissue-culture.md)
  - [6.2.5 Uses of Tissue Culture](https://senecalearning.com/en-gb/revision-notes/igcse/biology/edexcel-igcse/higher/6-2-5-uses-of-tissue-culture.md)
  - [6.2.6 Artificial Animal Cloning](https://senecalearning.com/en-gb/revision-notes/igcse/biology/edexcel-igcse/higher/6-2-6-artificial-animal-cloning.md)
  - [6.2.7 Exam-Style Questions - Mitosis & Cloning](https://senecalearning.com/en-gb/revision-notes/igcse/biology/edexcel-igcse/higher/6-2-7-exam-style-questions-mitosis-and-cloning.md)