Edexcel iGCSE Biology-4.24 & 4.25 Family Pedigrees- Study Notes- New Syllabus

Family Pedigrees

📝 Introduction

• A family pedigree (or pedigree chart) is a diagram that shows how an inherited characteristic or genetic disorder is passed through different generations of a family.
• It uses standard symbols to represent males, females and their relationships.
• Pedigrees help us trace the inheritance of a characteristic from parents to offspring.
• They can also help identify whether a trait is dominant or recessive and determine the possible genotypes of family members.
• Family pedigrees are widely used in genetics, medicine and genetic counselling.

🧬 What is a Family Pedigree?

• A family pedigree is a diagram that records the inheritance of a particular characteristic through several generations of a family.
• Instead of showing a genetic cross between two parents, it shows how a characteristic appears in an entire family over time.

Why are Pedigree Charts Used?

Pedigree charts help to:

• Trace the inheritance of a characteristic through generations.
• Identify affected and unaffected individuals.
• Determine whether a trait is dominant or recessive.
• Predict the chance of future children inheriting a characteristic.
• Study inherited genetic disorders in families.

Standard Symbols Used in Pedigree Charts

Understanding the symbols is the first step in interpreting a pedigree.

Understanding the Structure of a Pedigree

• A pedigree chart is usually arranged in generations.
• The oldest generation is shown at the top.
• Each new generation is shown below the previous one.
• Family relationships are connected using lines.

Generations

• Generations are labelled using Roman numerals.

Examples:

• Generation I
• Generation II
• Generation III

Individuals

• Each person within a generation is given a number.

Example:

Generation II

• II-1
• II-2
• II-3

• This numbering makes it easy to identify individuals.

🔍 How to Read a Pedigree Chart

When looking at a pedigree chart:

Step 1

Identify:

• Males (squares)
• Females (circles)

Step 2

Look for shaded symbols.

• Shaded individuals have the inherited characteristic being studied.
• Unshaded individuals do not have the characteristic.

Step 3

Identify family relationships.

Look for:

• Parents
• Children
• Brothers and sisters

Step 4

Observe how the characteristic appears in different generations.

Ask questions such as:

• Does the characteristic appear in every generation?
• Can unaffected parents have affected children?
• Are both males and females affected?
• These observations help identify the inheritance pattern.

🧬 Identifying Dominant Inheritance

A characteristic is likely to be dominant if:

• It usually appears in every generation.
• An affected person usually has at least one affected parent.
• Unaffected parents usually do not produce affected children.
• Both males and females can be affected.

Example Pattern

Generation I → Affected parent

                       ⬇

Generation II → Some affected children

                       ⬇

Generation III → Characteristic continues to appear

🧬 Identifying Recessive Inheritance

A characteristic is likely to be recessive if:

• It may skip generations.
• Two unaffected parents can have an affected child.
• Affected individuals may have unaffected parents.
• Both males and females can be affected.

Example Pattern

Generation I → Unaffected parents

                         ⬇

Generation II → Affected child appears

This suggests the parents were carriers.

Determining Possible Genotypes

Pedigree charts can also help predict genotypes.

Example

Let:

• B = Dominant allele
• b = Recessive allele

If the characteristic is recessive:

Affected individual:

• bb

Unaffected individual:

May be:

• BB
• Bb

The pedigree provides clues to decide which genotype is most likely.

🌸 Example

Suppose:

• A shaded symbol represents a recessive disorder.
• Two unaffected parents have one affected child.

What can we conclude?

• The parents must both carry the recessive allele.

Genotypes:

• Father → Bb
• Mother → Bb
• Affected child → bb

⚠️ Common Mistakes to Avoid

• Confusing males and females.
• Forgetting that shaded symbols represent affected individuals.
• Assuming every unaffected person is homozygous dominant.
• Ignoring information from earlier generations.
• Deciding whether a trait is dominant or recessive without enough evidence.

📊 Dominant vs Recessive Patterns in Pedigrees

📌 Quick Recap
– Shaded = affected, Unshaded = normal.
– Every generation → Dominant.
– Skips generation → Recessive.
– More males affected → X-linked.
✨ Trick: D = Dominant → Doesn’t skip. R = Recessive → Rare/hidden in some generations.