Inheritance Patterns » Family Pedigrees

What you'll learn this session

Study time: 30 minutes

How to read and interpret family pedigree charts
How to identify inheritance patterns from pedigrees
The symbols used in pedigree diagrams
How to determine if a trait is dominant, recessive, autosomal, or sex-linked
How to predict the probability of inheritance in future generations

Introduction to Family Pedigrees

Family pedigrees are diagrams that show how traits are passed down through generations of a family. They're like family trees, but with a focus on specific genetic traits or disorders. Scientists and doctors use pedigrees to track inheritance patterns and predict the likelihood of genetic conditions appearing in future family members.

Key Definitions:

Pedigree: A diagram showing genetic relationships within a family, tracking the inheritance of traits across generations.
Proband: The first affected family member who brings the family to medical attention.
Carrier: An individual who has a recessive allele for a genetic trait but doesn't show the trait themselves.
Genotype: The genetic makeup of an organism (e.g., AA, Aa, aa).
Phenotype: The observable characteristics resulting from the genotype.

👥 Pedigree Symbols

Pedigrees use standard symbols to represent people and their relationships:

Circle = Female
Square = Male
Filled symbol = Affected individual
Unfilled symbol = Unaffected individual
Half-filled symbol = Carrier (for recessive traits)
Horizontal line connecting symbols = Marriage/partnership
Vertical line from partnership = Offspring
Diagonal line through symbol = Deceased individual

📊 Reading Pedigrees

When analysing a pedigree chart:

Start at the top (oldest generation)
Work down through generations (usually labelled with Roman numerals: I, II, III)
Individuals in each generation are numbered from left to right
Look for patterns of affected individuals
Consider whether males or females are more commonly affected
Check if the trait appears in every generation or skips generations

Inheritance Patterns in Pedigrees

By carefully examining a pedigree, we can determine whether a trait follows autosomal dominant, autosomal recessive, X-linked dominant, or X-linked recessive inheritance. Each pattern has distinctive characteristics that help us identify it.

Autosomal Dominant Inheritance

In autosomal dominant inheritance, a single copy of the dominant allele is enough to express the trait. These traits appear in every generation and affect males and females equally.

💡 Key Features

The trait appears in every generation
Affected individuals usually have an affected parent
Males and females are equally affected
Approximately 50% of offspring from an affected parent will show the trait
Unaffected parents don't have affected children

🩹 Examples

Huntington's disease
Achondroplasia (a form of dwarfism)
Marfan syndrome
Some forms of hypercholesterolemia
Polydactyly (extra fingers or toes)

Autosomal Recessive Inheritance

In autosomal recessive inheritance, two copies of the recessive allele are needed to express the trait. These traits often skip generations and require both parents to be carriers or affected.

💡 Key Features

The trait often skips generations
Affected individuals usually have unaffected parents (who are carriers)
Males and females are equally affected
More common in families with consanguinity (related parents)
Approximately 25% of offspring from two carrier parents will show the trait

🩹 Examples

Cystic fibrosis
Sickle cell anaemia
Tay-Sachs disease
Phenylketonuria (PKU)
Albinism

Sex-Linked Inheritance

Sex-linked traits are carried on the sex chromosomes (X or Y). X-linked traits are much more common because the Y chromosome carries fewer genes. These patterns show distinctive male-female differences in inheritance.

👩 X-Linked Recessive

Males are more commonly affected
Females can be carriers without showing the trait
No male-to-male transmission
Affected males pass the allele to all their daughters (who become carriers)
Examples: Haemophilia, Red-green colour blindness, Duchenne muscular dystrophy

👨 X-Linked Dominant

More females than males are affected
Affected males pass the trait to all their daughters but none of their sons
No male-to-male transmission
More severe in males than females
Examples: Vitamin D-resistant rickets, Rett syndrome

👦 Y-Linked

Only males are affected
All sons of affected males will inherit the trait
Daughters are never affected
Very rare pattern
Example: Some forms of hearing loss, certain fertility issues

Case Study Focus: The Royal Haemophilia

One of the most famous pedigrees in history traces haemophilia through Queen Victoria's descendants. Queen Victoria was a carrier of haemophilia, an X-linked recessive disorder that affects blood clotting. The condition spread to royal families across Europe through her children.

The pedigree shows classic X-linked recessive inheritance:

The disorder appeared in males but was carried by females
No male-to-male transmission occurred
Affected males did not pass the condition to their sons
Daughters of affected males were all carriers

This royal pedigree helped scientists understand X-linked inheritance patterns before the discovery of DNA's structure.

Analysing Pedigrees: A Step-by-Step Approach

When you're given a pedigree to analyse in an exam, follow these steps to determine the inheritance pattern:

Check for gender bias: Are more males or females affected? If males are predominantly affected, suspect X-linked recessive. If it affects both genders equally, it's likely autosomal.
Look for generation patterns: Does the trait appear in every generation (dominant) or skip generations (recessive)?
Check for carrier evidence: Are there unaffected individuals who have affected children? This suggests carriers of a recessive trait.
Look for male-to-male transmission: If present, the trait cannot be X-linked.
Consider the frequency: Is the trait rare or common in the pedigree? Dominant traits tend to appear more frequently.

Calculating Probabilities from Pedigrees

Once you've identified the inheritance pattern, you can calculate the probability of a trait appearing in future generations:

🎲 Probability Calculations

For autosomal dominant traits:

If one parent is affected (heterozygous) and one is unaffected: 50% chance for each child
If one parent is homozygous dominant and one is unaffected: 100% chance for each child

For autosomal recessive traits:

If both parents are carriers: 25% chance of affected child, 50% chance of carrier, 25% chance of unaffected non-carrier
If one parent is affected and one is a carrier: 50% chance of affected child, 50% chance of carrier

For X-linked recessive traits:

If mother is a carrier and father is unaffected: 50% chance for sons to be affected, 50% chance for daughters to be carriers
If father is affected and mother is unaffected: All daughters will be carriers, all sons will be unaffected

Common Mistakes to Avoid

When analysing pedigrees, be careful to avoid these common errors:

Confusing dominant and recessive patterns
Forgetting that carriers don't show the trait but can pass it on
Overlooking the possibility of new mutations
Assuming that all genetic disorders follow simple inheritance patterns
Forgetting that some traits have incomplete penetrance (not everyone with the genotype shows the phenotype)

Real-World Application: Genetic Counselling

Pedigree analysis is a crucial tool in genetic counselling. Genetic counsellors work with families to:

Identify inheritance patterns of genetic conditions
Calculate the risk of having a child with a genetic disorder
Help families make informed decisions about family planning
Recommend appropriate genetic testing
Provide emotional support to families affected by genetic conditions

Understanding how to interpret pedigrees is an essential skill for these healthcare professionals.

🧠 Test Your Knowledge!