Model Answer
0 min readIntroduction
Haemophilia is a rare, inherited bleeding disorder in which the blood doesn't clot normally. This is typically due to a deficiency in one of the clotting factors. The most common type is haemophilia A, caused by a deficiency in clotting factor VIII. Crucially, haemophilia is an X-linked recessive genetic condition. This means the gene responsible for the condition is located on the X chromosome, and a female needs two copies of the mutated gene to express the trait, while a male only needs one. Understanding this inheritance pattern is key to determining the probability of affected offspring. This answer will calculate the percentage of haemophilic sons and daughters when a mother is a carrier and the father is unaffected.
Understanding X-linked Recessive Inheritance
X-linked recessive inheritance means that males are more likely to be affected by the condition because they have only one X chromosome. If they inherit the affected X chromosome, they will express the trait. Females have two X chromosomes, so they need to inherit the affected gene on both X chromosomes to express the trait. If a female inherits one affected gene, she becomes a carrier – she doesn’t show symptoms but can pass the gene on to her children.
Scenario: Carrier Mother, Unaffected Father
Let's denote the normal allele as 'X' and the haemophilic allele as 'Xh'. A carrier mother has the genotype X Xh, and an unaffected father has the genotype X Y (males only have one X chromosome).
Punnett Square Analysis
To determine the probability of affected offspring, we can use a Punnett square:
| X | Y | |
|---|---|---|
| X | XX (Unaffected Daughter) | XY (Unaffected Son) |
| Xh | XhX (Carrier Daughter) | XhY (Haemophilic Son) |
Calculating Percentages
- Haemophilic Sons: From the Punnett square, one out of four offspring (25%) is XhY, meaning they will be haemophilic. Therefore, the percentage of haemophilic sons is 25%.
- Haemophilic Daughters: None of the daughters in this scenario will be haemophilic (XX or XhX). They will either be unaffected (XX) or carriers (XhX). Therefore, the percentage of haemophilic daughters is 0%.
Important Considerations
This calculation assumes that the haemophilia gene is the only factor influencing the outcome. In reality, spontaneous mutations can occur, and other genetic factors might play a role. Furthermore, the severity of haemophilia can vary depending on the specific mutation in the clotting factor gene.
Severity of Haemophilia
Haemophilia is classified based on the level of clotting factor in the blood:
- Severe Haemophilia: Clotting factor level less than 1%
- Moderate Haemophilia: Clotting factor level 1-5%
- Mild Haemophilia: Clotting factor level 6-40%
Conclusion
In conclusion, when a mother is a carrier for haemophilia and the father is unaffected, there is a 25% chance of having a haemophilic son and a 0% chance of having a haemophilic daughter. This is due to the X-linked recessive nature of the disease. Understanding these inheritance patterns is crucial for genetic counselling and family planning for individuals at risk. Further advancements in gene therapy offer potential future treatments for haemophilia, but currently, management focuses on replacing the missing clotting factor.
Answer Length
This is a comprehensive model answer for learning purposes and may exceed the word limit. In the exam, always adhere to the prescribed word count.