Inheritance » Sex-Linked Inheritance

What you'll learn this session

Study time: 30 minutes

Understand what sex-linked inheritance means and how it differs from normal inheritance
Learn about X-linked and Y-linked traits and their patterns
Explore examples like colour blindness and haemophilia
Master genetic crosses involving sex-linked characteristics
Understand why males are more affected by X-linked disorders
Apply knowledge to real-world genetic counselling scenarios

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Introduction to Sex-Linked Inheritance

Sex-linked inheritance is a fascinating area of genetics where certain traits are controlled by genes located on the sex chromosomes (X and Y). Unlike normal inheritance patterns, these traits show different patterns in males and females because of the way sex chromosomes are inherited.

Key Definitions:

Sex-linked gene: A gene located on either the X or Y chromosome that controls a particular trait.
X-linked trait: A characteristic controlled by a gene on the X chromosome.
Y-linked trait: A characteristic controlled by a gene on the Y chromosome (rare in humans).
Carrier: An individual (usually female) who has one copy of a recessive allele but doesn't show the trait.
Hemizygous: Having only one copy of a gene (males for X-linked genes).

♂ Male Pattern

Males have XY chromosomes, so they only have one X chromosome. This means they only need one copy of a recessive X-linked allele to express the trait. Males cannot be carriers - they either have the trait or they don't.

♀ Female Pattern

Females have XX chromosomes, so they have two X chromosomes. They need two copies of a recessive X-linked allele to express the trait. Females can be carriers if they have one dominant and one recessive allele.

Understanding X-Linked Inheritance

X-linked inheritance is the most common type of sex-linked inheritance in humans. The X chromosome carries many important genes, whilst the Y chromosome is much smaller and carries fewer genes.

How X-Linked Traits Work

When a gene is located on the X chromosome, it follows a unique inheritance pattern. Because males only have one X chromosome, they're more likely to be affected by X-linked recessive disorders. Females, having two X chromosomes, are usually protected because one normal copy can often compensate for a faulty one.

🔴 Affected Male

Has the recessive allele on his single X chromosome. Will definitely show the trait and pass it to all daughters.

🔵 Carrier Female

Has one normal and one recessive allele. Usually doesn't show the trait but can pass it to children.

🟢 Normal Individual

Has normal alleles and won't pass on the recessive trait to offspring.

Case Study Focus: Colour Blindness

Red-green colour blindness affects about 8% of men but only 0.5% of women. This huge difference shows the power of X-linked inheritance. The gene for colour vision is on the X chromosome, so men only need one faulty copy to be colour blind, whilst women need two faulty copies - much less likely to happen!

Common X-Linked Disorders

Several important human disorders follow X-linked inheritance patterns. Understanding these helps us see how genetics affects real families.

Haemophilia

Haemophilia is a bleeding disorder where blood doesn't clot properly. It's caused by a recessive allele on the X chromosome. This condition famously affected European royal families in the 19th and 20th centuries.

Inheritance Pattern:

Affected fathers cannot pass haemophilia to sons (sons get Y chromosome from father)
All daughters of affected fathers are carriers
Carrier mothers have a 50% chance of passing the allele to each child
Sons who inherit the allele will have haemophilia
Daughters who inherit the allele will be carriers (unless father also affected)

🤔 Why Males Are More Affected

Males are hemizygous for X-linked genes - they only have one copy. If that copy is faulty, there's no backup. It's like having only one spare tyre - if it's flat, you're stuck!

💪 Female Protection

Females usually have protection through X-inactivation. In each cell, one X chromosome is randomly switched off, so if one carries a faulty gene, the other can often compensate.

Genetic Crosses and Punnett Squares

Working out sex-linked inheritance requires special Punnett squares that show the sex chromosomes. Let's look at how to predict outcomes for X-linked traits.

Setting Up Sex-Linked Crosses

When drawing genetic crosses for sex-linked traits, we write the alleles as superscripts on the X chromosome. For example, if we're looking at colour blindness where 'C' = normal vision and 'c' = colour blind:

Normal vision male: X^CY
Colour blind male: X^cY
Normal vision female: X^CX^C
Carrier female: X^CX^c
Colour blind female: X^cX^c

Worked Example: Carrier Mother × Normal Father

Mother (carrier): X^CX^c × Father (normal): X^CY

Results: 25% normal daughters, 25% carrier daughters, 25% normal sons, 25% colour blind sons

This shows why X-linked traits often 'skip generations' - going from affected grandfather to affected grandson through a carrier daughter.

Y-Linked Inheritance

Y-linked inheritance is much simpler but also much rarer. Only males can inherit Y-linked traits and they pass them to all their sons.

👤 Father to Son Only

Y-linked traits pass directly from father to son with no variation. All sons of an affected male will have the trait and no daughters can inherit it.

🧑 Examples in Humans

Most Y-linked genes in humans control male development and fertility. The SRY gene (sex-determining region Y) is crucial for male development.

Genetic Counselling and Sex-Linked Disorders

Understanding sex-linked inheritance is crucial for genetic counselling. Families need to understand the risks and patterns of inheritance for informed decision-making.

Risk Assessment

Genetic counsellors help families understand:

The chance of having an affected child
Whether someone is a carrier
How the condition might affect family planning
Available testing and treatment options

For X-linked disorders, the risks depend heavily on the sex of the child and the genetic status of the parents. This makes family trees (pedigrees) particularly important for tracking inheritance patterns.

Real-World Application: Duchenne Muscular Dystrophy

This severe muscle-wasting disease affects about 1 in 3,500 boys. It's X-linked recessive, so affected boys usually have unaffected parents. The mother is typically a carrier and there's a 50% chance each son will be affected. Understanding this pattern helps families prepare and make informed choices about having more children.

Key Patterns to Remember

Sex-linked inheritance creates distinctive family patterns that are different from autosomal inheritance.

↓ Male to Female

Affected fathers pass X-linked traits to ALL daughters (who become carriers) but to NO sons.

↗ Skipping Generations

X-linked traits often skip generations, going from affected grandfather to affected grandson.

♂ More Males Affected

X-linked recessive disorders affect far more males than females in the population.

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