Edexcel iGCSE Biology-4.22 Polygenic Inheritance- Study Notes- New Syllabus

Polygenic Inheritance

📝 Introduction

• Some inherited characteristics are controlled by only one gene, but many characteristics are controlled by several genes working together.
• When a characteristic is controlled by more than one gene, it is called polygenic inheritance.
• Most phenotypic features in humans, animals and plants are the result of polygenic inheritance.
• Polygenic inheritance produces a wide range of variations, so individuals show many different forms of the same characteristic.

🧬 What is Polygenic Inheritance?

Definition

• Polygenic inheritance is the inheritance of a characteristic that is controlled by two or more genes.
• Each gene contributes a small effect to the final characteristic.
• The combined effect of all these genes determines the phenotype.

Simple Definition

• Polygenic inheritance is when several genes work together to control a single characteristic.

Why is it Called “Polygenic”?

The word polygenic can be divided into two parts:

• Poly = Many
• Genic = Genes

So, polygenic means “controlled by many genes.”

How Polygenic Inheritance Works

• Unlike a characteristic controlled by a single gene, polygenic characteristics are influenced by many genes at the same time.
• Each gene adds a small contribution.
• The final phenotype depends on the combined effect of all these genes.

Simple Flow

Many Genes → Combined Effects → One Characteristic

Features of Polygenic Inheritance

• Controlled by two or more genes.
• Each gene has a small effect on the characteristic.
• No single gene completely determines the phenotype.
• Produces continuous variation rather than only a few distinct categories.
• Individuals show many different forms of the same characteristic.

What is a Phenotypic Feature?

Definition

• A phenotypic feature is any observable characteristic of an organism.
• It is what we can see or measure.

Examples include:

• Height
• Skin colour
• Eye colour
• Body mass
• Hair colour

Phenotypic features are produced by the interaction of genes and, in some cases, the environment.

Most Phenotypic Features are Polygenic

• Many characteristics do not have only two forms.
• Instead, they show a wide range of differences between individuals.
• This happens because they are controlled by several genes.

Examples

• Some people are very tall.
• Some are of average height.
• Some are shorter.

There are many heights in between, not just “tall” or “short.”

This continuous range is a result of polygenic inheritance.

🌸 Examples of Polygenic Characteristics

1. Human Height

• Height is controlled by many genes.
• Each gene contributes a small amount to the person’s final height.

As a result:

• Very short individuals
• Short individuals
• Medium height individuals
• Tall individuals
• Very tall individuals

All are possible.

Nutrition and health during growth can also affect final height.

2. Skin Colour

• Skin colour is influenced by several genes.
• Each gene affects the amount of melanin produced.
• Different combinations of these genes produce many different skin tones.
• Instead of only light or dark skin, there is a continuous range of skin colours.

3. Body Mass

• Body mass is influenced by many genes.
• Genes affect:
  • Growth
  • Fat storage
  • Muscle development

Lifestyle factors such as diet and exercise also influence body mass.

4. Eye Colour

• Eye colour is mainly controlled by several genes.
• Different gene combinations produce many eye colours such as:
  • Brown
  • Hazel
  • Green
  • Blue
  • Grey

📈 Continuous Variation

What is Continuous Variation?

• Continuous variation means there are many possible values for a characteristic rather than only a few categories.

Examples:

• Height can range from very short to very tall.
• Skin colour shows many shades.
• Body mass varies continuously.

There are no clear boundaries between the different forms.

Polygenic Inheritance vs Single Gene Inheritance

Single Gene Inheritance

• Controlled by one gene.
• Usually produces a small number of distinct phenotypes.
• Characteristics fall into clear categories.

Examples

• Blood group
• Ability to roll the tongue (commonly used as a simple genetic example)
• Some inherited disorders

Polygenic Inheritance

• Controlled by many genes.
• Produces a wide range of phenotypes.
• Characteristics vary continuously.

Examples

• Height
• Skin colour
• Body mass
• Eye colour

🌍 Effect of the Environment

• Many polygenic characteristics are also affected by environmental factors.
• This means the final phenotype depends on both genes and the environment.

Examples

Height

• Genes determine the potential height.
• However, poor nutrition or illness during childhood may reduce growth.

Body Mass

• Genes influence body mass.
• Diet and physical activity also affect body mass.

Skin Colour

• Genes determine natural skin colour.
• Sunlight can increase melanin production, making the skin darker.

📊 Single Gene vs Polygenic Inheritance

📊 Examples of Polygenic Characteristics

🌟 Why is Polygenic Inheritance Important?

• Polygenic inheritance:
  • Produces variation within a population.
  • Explains why individuals look different from one another.
  • Creates a wide range of phenotypes.
  • Helps populations adapt to changing environments.
  • Contributes to biodiversity.

📌 Quick Recap
– Polygenic inheritance = many genes control one trait.
– Produces continuous variation (smooth range, not categories).
– Examples: height, skin color, weight, eye color.
– Contrast: Monogenic inheritance → single gene, discrete categories (like blood groups).
✨ Trick: Poly = Many → Polygenic = Many genes for one trait.