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, most commonly Factor VIII (Haemophilia A) or Factor IX (Haemophilia B). Crucially, haemophilia is an X-linked recessive genetic condition. This means the gene responsible for the disorder is located on the X chromosome, and a female needs two copies of the defective gene to express the trait, while a male only needs one. Understanding the inheritance pattern is vital for genetic counseling and predicting the risk of transmission to future generations. This answer will detail the possible outcomes when a haemophilic man and a normal woman have children.
Understanding X-linked Recessive Inheritance
X-linked recessive inheritance differs from autosomal inheritance. Humans have 23 pairs of chromosomes, including one pair of sex chromosomes: XX for females and XY for males. Since males have only one X chromosome, any recessive gene present on that chromosome will be expressed. Females, having two X chromosomes, must inherit the recessive gene on both chromosomes to exhibit the trait. If a female carries only one copy of the recessive gene, she is considered a carrier – she doesn’t show symptoms but can pass the gene on to her children.
Genotypes of the Parents
Let's denote the normal allele for the clotting factor gene as 'XH' and the haemophilic allele as 'Xh'.
- Haemophilic Man: Since haemophilia is X-linked recessive, a male with haemophilia must have the genotype XhY. He has the haemophilic allele on his single X chromosome and a Y chromosome.
- Normal Woman: A normal woman can have two possible genotypes: XHXH (homozygous normal) or XHXh (heterozygous carrier). We will analyze both scenarios.
Scenario 1: Normal Woman is Homozygous (XHXH)
In this case, the woman has two normal alleles. The Punnett square is as follows:
| Xh | Y | |
|---|---|---|
| XH | XHXh | XHY |
| XH | XHXh | XHY |
Results:
- All daughters (XHXh) will be carriers of haemophilia.
- All sons (XHY) will be normal.
Therefore, in this scenario, there is a 0% chance of having a son with haemophilia and a 0% chance of having a daughter with haemophilia.
Scenario 2: Normal Woman is a Carrier (XHXh)
This is the more common scenario. The Punnett square is as follows:
| Xh | Y | |
|---|---|---|
| XH | XHXh | XHY |
| Xh | XhXh | XhY |
Results:
- Daughters:
- 25% (XHXh) will be carriers.
- 25% (XhXh) will have haemophilia.
- Sons:
- 25% (XHY) will be normal.
- 25% (XhY) will have haemophilia.
Therefore, in this scenario, there is a 25% chance of having a son with haemophilia and a 25% chance of having a daughter with haemophilia.
Why Haemophilia is More Common in Males
As explained earlier, males only have one X chromosome. Therefore, if they inherit the haemophilic allele (Xh), they will express the trait. Females, with two X chromosomes, need to inherit the haemophilic allele on both chromosomes to be affected. This makes haemophilia significantly more prevalent in males.
Conclusion
In conclusion, the outcome of a cross between a haemophilic man (X<sup>h</sup>Y) and a normal woman depends on whether the woman is homozygous normal (X<sup>H</sup>X<sup>H</sup>) or a carrier (X<sup>H</sup>X<sup>h</sup>). If the woman is homozygous normal, all sons will be normal and all daughters will be carriers. If the woman is a carrier, there is a 25% chance of having a son with haemophilia and a 25% chance of having a daughter with haemophilia. The X-linked recessive nature of haemophilia explains its higher incidence in males. Genetic counseling is crucial for families with a history of haemophilia to understand the risks and make informed decisions.
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.