3.1. Explain sex-linked inheritance

This guide will help you answer 3.1. Explain sex-linked inheritance.

Sex-linked inheritance refers to how certain traits or conditions are passed from parents to offspring through genes located on the sex chromosomes. Humans have 23 pairs of chromosomes, with one pair being sex chromosomes—X and Y. These chromosomes determine an individual’s biological sex: XX for females and XY for males. The X and Y chromosomes also carry specific genes, and traits linked to these genes are called sex-linked traits.

This type of inheritance is significant because some diseases and conditions are more likely to affect one sex than the other due to differences in how the X and Y chromosomes function.

What are Chromosomes and Genes?

Chromosomes are structures made of long strands of DNA. DNA contains the instructions for building and maintaining the body, organised into units called genes. Each gene provides specific instructions, like coding for proteins that influence physical characteristics (e.g., eye colour) or bodily functions.

Sex chromosomes differ between males and females.

• X Chromosome: Contains many genes unrelated to sex determination, such as those that control blood clotting or vision.
• Y Chromosome: Mostly smaller and carries fewer functional genes, many of which are directly related to male sexual development.

Because males have only one X chromosome, they are more likely to express traits linked to genes on the X chromosome, whether the gene is dominant (stronger) or recessive (weaker). Females with two X chromosomes have the potential for one gene’s copy to compensate for a defective one.

Sex-Linked Traits Explained

Sex-linked traits are determined by genes found on the X or Y chromosome. Most sex-linked traits are carried on the X chromosome because it has more genes than the Y chromosome. The inheritance pattern is influenced by whether the trait is dominant or recessive and whether it is carried on the X or Y chromosome.

X-Linked Inheritance

X-linked inheritance involves genes located on the X chromosome. These traits are inherited differently in males and females because males only have one X chromosome while females have two. There are two main types of X-linked inheritance:

X-Linked Dominant:

• A single copy of a dominant gene on the X chromosome is enough to cause the trait.
• Females (XX) can inherit the condition from one affected parent. Males (XY) inherit it if their one X chromosome carries the faulty gene.
• Examples include certain forms of rickets and Rett syndrome.

X-Linked Recessive:

• A recessive gene on the X chromosome causes this condition.
• Females (XX) usually need two copies of the gene (one from each parent) to express the condition, though they can be carriers with only one defective gene.
• Males (XY), with only one X chromosome, express the condition if their X chromosome carries the gene.
• Examples include haemophilia, red-green colour blindness, and Duchenne muscular dystrophy.

Y-Linked Inheritance

Y-linked traits are caused by genes found on the Y chromosome and are passed only from fathers to sons. Y-linked inheritance is relatively rare because the Y chromosome has fewer genes. Traits or disorders linked to the Y chromosome typically affect male sexual development, such as a condition called Y-linked infertility.

Why Sex-Linked Inheritance Differs Between Males and Females

Males and females inherit sex-linked traits differently because of their sex chromosomes.

• Males (XY):
• Males have only one X chromosome. This means they do not have a “backup” X if the gene on their X chromosome is faulty.
• Any gene on the X chromosome, whether recessive or dominant, directly influences the male’s traits or health.
• Females (XX):
• Females have two X chromosomes. One healthy copy of a gene on one X chromosome can often mask or offset a faulty gene on the other.
• Females are more likely to be carriers of X-linked conditions but less likely to suffer from them compared to males.

Common Examples of X-Linked Inheritance

There are several conditions caused by X-linked inheritance, including:

1. Haemophilia

Haemophilia is a blood disorder where the blood fails to clot properly. It is caused by a recessive gene located on the X chromosome.

• Males: If they inherit the faulty X gene, they will have haemophilia because they lack a second X chromosome to compensate.
• Females: They need two copies of the faulty gene to develop haemophilia but may carry the condition with only one faulty gene.

Carriers can pass the condition to their offspring:

• Sons have a 50% chance of inheriting the condition if the mother is a carrier.
• Daughters have a 50% chance of being carriers.

2. Red-Green Colour Blindness

This condition affects the ability to distinguish between red and green hues. It is caused by a recessive gene on the X chromosome.

• Males: With one faulty X chromosome, the condition develops because they lack another X chromosome to compensate.
• Females: Must inherit two faulty X chromosomes to develop the condition.

Since the condition is common in males, colour blindness affects around 8% of the male population, compared to less than 1% of females.

3. Duchenne Muscular Dystrophy

Duchenne muscular dystrophy causes progressive muscle weakness and wasting due to mutations in the dystrophin gene on the X chromosome.

• Males: Develop the condition if their X chromosome has the faulty gene.
• Females: Rarely have symptoms but can be carriers, passing the gene to their children.

The Role of Carriers

Carriers are individuals who have one copy of a faulty gene but do not express the condition themselves. In X-linked inheritance, carriers are typically females because they have two X chromosomes. Male carriers do not exist as they either have the condition or they don’t.

Female carriers can pass the faulty gene to their children:

• Sons: Are at risk of inheriting the condition.
• Daughters: May become carriers or inherit the condition if their other X chromosome is also faulty.

Punnett Squares and Predicting Inheritance

A Punnett square is a tool used to predict the chances of inheriting specific traits, including sex-linked traits. This method involves crossing genetic information from the parents to show possible outcomes for their children.

For example:
If a mother is a carrier for an X-linked condition and the father is unaffected:

• There is a 50% chance they will have a son with the condition.
• There is a 50% chance they will have a daughter who is a carrier.

If the father has a Y-linked condition:

• Only male offspring will inherit the condition because only sons receive a Y chromosome from their father.

Dosage Compensation and X-Inactivation

In females, one of the two X chromosomes in each cell becomes inactive through a process called X-inactivation. This prevents females from producing twice as many proteins from their two X chromosomes compared to males (who have only one X chromosome). X-inactivation happens randomly in each cell, creating a mix of active X chromosomes from both parents.

This process means some X-linked traits may not show as strongly in females because the faulty gene on one X chromosome might be “switched off.”

Other Patterns of Sex-Linked Inheritance

Sex-linked inheritance isn’t limited to just diseases. It can include other traits like hair patterning or certain physical characteristics. Generally, these are less studied because they do not pose significant health concerns.

Why It Matters in Health and Social Care

As a health and social care worker, understanding sex-linked inheritance helps you better support individuals living with genetic conditions. You may be involved in providing information to families, coordinating care for those with inherited disorders, or advocating for genetic counselling services.

Knowledge of sex-linked inheritance can also guide how you approach conversations about family health histories, empowering patients and families to make informed decisions about genetic risks.

Examples of good practices include:

• Encouraging individuals with a family history of genetic disorders to seek genetic testing.
• Supporting the emotional well-being of individuals or families affected by genetic conditions.
• Collaborating with other professionals, like genetic counsellors or medical specialists, for comprehensive care.

By recognising how genes and inheritance patterns work, you can approach your role with greater confidence and provide compassionate, informed care.