Understanding Haemophilia as a Genetic Disorder

Haemophilia is a well-known genetic disorder that affects the blood's ability to clot properly. People with haemophilia may experience prolonged bleeding after an injury or surgery, and in severe cases, spontaneous bleeding into joints, muscles, or organs. Understanding the genetic basis of haemophilia helps us classify it correctly among different types of inheritance patterns.

Sex-Linked Inheritance Explained

When a genetic disorder is described as "sex-linked," it means that the gene responsible for the disorder is located on one of the sex chromosomes, either the X chromosome or the Y chromosome. Since the Y chromosome is much smaller and carries fewer genes than the X chromosome, most sex-linked disorders are X-linked.

• Males have one X and one Y chromosome (XY).
• Females have two X chromosomes (XX).

Haemophilia is caused by a mutation in a gene located on the X chromosome. This is why it affects males and females differently.

Why Haemophilia is Recessive

A genetic trait or disorder is described as "recessive" if two copies of the mutated gene are needed to express the condition. If the disorder is on an autosome (a non-sex chromosome), an individual must inherit two copies of the recessive allele (one from each parent) to have the disorder. However, for X-linked disorders, the concept of recessive needs careful consideration, especially in males.

• In females (XX), a recessive X-linked disorder requires both X chromosomes to carry the mutated gene (\(X^hX^h\)). If only one X chromosome carries the mutation (\(X^HX^h\)), the individual is typically a carrier and does not show symptoms because the healthy gene on the other X chromosome is dominant.
• In males (XY), there is only one X chromosome. Therefore, if the single X chromosome carries the mutated gene (\(X^hY\)), the male will have the disorder, even if the trait is recessive in females. This is because there is no second X chromosome with a dominant healthy gene to mask the effect of the recessive gene.

Since a single copy of the mutated gene on the X chromosome is sufficient to cause the disorder in males, and two copies are needed for females, the disorder behaves recessively relative to the dominant healthy allele. The condition is generally much more common and severe in males due to their single X chromosome.

Inheritance Patterns of Haemophilia

Let's look at how haemophilia can be passed down:

This pattern clearly shows that the disorder follows the X chromosome and appears when the recessive allele is present on the X chromosome in males or on both X chromosomes in females (though affected females are rare due to requiring an affected father and at least a carrier mother).

Comparing Inheritance Types

Let's consider why the other options are less likely:

• Sex-linked dominant: A single copy of the mutated gene on an X chromosome would cause the disorder in both males and females. Affected fathers would pass the disorder to all their daughters. This is not the typical pattern seen in haemophilia.
• Autosomal dominant: The gene would be on a non-sex chromosome, and a single copy of the mutated gene would cause the disorder. It would affect males and females equally, and there would be a 50% chance of passing the disorder to any child from an affected parent. This differs from the sex-linked pattern of haemophilia.
• Autosomal recessive: The gene would be on a non-sex chromosome, and two copies of the mutated gene would be needed. It would affect males and females equally, and affected individuals could have unaffected parents (who are carriers). While it affects males and females equally, the inheritance pattern is not linked to sex chromosomes as observed with haemophilia.

Based on the characteristic inheritance pattern where the disorder is carried on the X chromosome and expressed when a recessive allele is present (especially notably in males), haemophilia is classified as a sex-linked recessive disorder.

Revision Table: Haemophilia Inheritance

Additional Information on Haemophilia

Haemophilia primarily refers to two main types:

• Haemophilia A: Caused by a deficiency in clotting Factor VIII. This is the most common type.
• Haemophilia B: Caused by a deficiency in clotting Factor IX. Also known as Christmas disease.

Both Haemophilia A and B are inherited in an X-linked recessive pattern. There is also Haemophilia C (Factor XI deficiency), which has a different inheritance pattern (autosomal recessive) and is generally milder.

Understanding the sex-linked recessive nature is crucial for genetic counselling and predicting inheritance risks within families affected by Haemophilia A or B.