Haemophilia is a Genetic Disorder Where Blood Clotting Fails
- Imagine every time you get a simple cut it doesn’t stop bleeding.
- For individuals with haemophilia, this is a constant reality.
- The condition occurs when the body cannot produce enough Factor VIII (haemophilia A) or Factor IX (haemophilia B), both essential for blood clotting.
Definition
Haemophilia
Haemophilia is a disorder where blood fails to clot properly due to a deficiency in a clotting factor.
Haemophilia is a Sex-Linked Disorder Passed on Through the X Chromosome
- The gene for Factor VIII is located on the X chromosome, making haemophilia a sex-linked recessive disorder.
- Males (XY): Have haemophilia if their X chromosome carries the mutated allele.
- Females (XX): Only have haemophilia if they inherit two mutated alleles (very rare). They are more often carriers and may pass the disorder to their children.
Carriers usually produce enough Factor VIII to remain unaffected.
Males are More Likely to Have Haemophilia
- Males only inherit one X chromosome.
- If it carries the haemophilia mutation (XhX^hXh), they develop the disorder because they lack a second X chromosome to compensate.
- In females, the presence of a normal XHX^HXH allele masks the mutation unless both X chromosomes carry the defective gene.
Punnett Grids Show How Haemophilia is Inherited
- A carrier mother (XHXhX^H X^hXHXh) and a normal father (XHYX^H YXHY) have the following probabilities for their offspring:
- 25% chance of a son with haemophilia (XhYX^h YXhY).
- 25% chance of a daughter who is a carrier (XHXhX^H X^hXHXh).
- 25% chance of a son without haemophilia (XHYX^H YXHY).
- 25% chance of a daughter without the defective allele (XHXHX^H X^HXHXH).
| $X^H$ | $X^h$ | |
|---|---|---|
| $X^H$ | $X^HX^H$ | $X^HX^h$ |
| $Y$ | $X^HY$ | $X^hY$ |
When studying sex-linked traits, always show alleles as superscripts on the X chromosome (e.g., $X^H$ or $X^h$).
Haemophilia is Rare, Affecting 1 in 10,000 Males Globally
- Haemophilia is significantly rarer in females because they require two defective alleles.
- The theoretical frequency of haemophilia in females is about 1 in 100 million due to this requirement.
- A common misconception is that females cannot have haemophilia.
- While rare, females can have the disorder if they inherit two recessive alleles ($X^hX^h$).
Modern Treatments Help Manage Haemophilia
- Haemophilia is treated using:
- Factor Replacement Therapy: Regular injections of the missing clotting factor.
- Gene Therapy: Emerging technology that offers potential permanent cures by correcting the faulty gene.
- How do ethical considerations influence decisions about genetic testing for disorders like haemophilia?
- Should carriers be required to disclose their status to potential partners?
- Can you explain why haemophilia is more common in males than females?
- Try drawing a Punnett grid to illustrate your answer.
