Model Answer
0 min readIntroduction
Haemophilia is a rare, inherited bleeding disorder in which the blood doesn't clot normally. This is most commonly caused by a deficiency in clotting factor VIII (Haemophilia A) or factor IX (Haemophilia B), both of which are encoded by genes located on the X chromosome. Because of this X-linked inheritance pattern, haemophilia affects males more frequently than females. Understanding the inheritance pattern is crucial for genetic counseling and predicting the risk of transmission to future generations. This answer will detail the genetic outcome of a cross between a haemophilic woman and a normal man, outlining the probabilities of affected and carrier offspring.
Understanding X-linked Recessive Inheritance
X-linked recessive inheritance means that the gene responsible for the trait is located on the X chromosome. Females have two X chromosomes (XX), while males have one X and one Y chromosome (XY). A female must inherit two copies of the recessive allele to express the trait, while a male only needs to inherit one copy, as there is no corresponding allele on the Y chromosome to mask its effect.
Genotypes of the Parents
Let 'XH' represent the normal allele for clotting factor and 'Xh' represent the allele for haemophilia.
- Haemophilic woman: XhXh (She has two copies of the haemophilia allele)
- Normal man: XHY (He has one normal allele on his X chromosome and a Y chromosome)
Punnett Square
To determine the possible genotypes and phenotypes of their offspring, we can construct a Punnett square:
| XH | Y | |
|---|---|---|
| Xh | XHXh | XhY |
| Xh | XHXh | XhY |
Analysis of Offspring Genotypes and Phenotypes
From the Punnett square, we can see the following possible genotypes and phenotypes:
- XHXh: Daughter, carrier of haemophilia (phenotypically normal, but can pass the allele to her children). Probability: 50%
- XhY: Son, haemophilic (affected by the disease). Probability: 50%
Implications for Sons and Daughters
This cross demonstrates that:
- All daughters will be carriers of the haemophilia allele. They will not exhibit the disease themselves, but they have a 50% chance of passing the allele to their children.
- All sons will be affected by haemophilia, as they inherit the single Xh allele from their mother and have no corresponding allele on their Y chromosome.
Why Haemophilia is More Common in Males
Haemophilia is significantly more common in males because they only have one X chromosome. If a male inherits the recessive allele on his X chromosome, he will express the trait. Females, having two X chromosomes, must inherit two copies of the recessive allele to be affected, making it less likely. Approximately 1 in 5,000 males are born with haemophilia, compared to about 1 in 25,000 females (data as of 2023, based on World Federation of Hemophilia estimates).
Genetic Counseling
Understanding this inheritance pattern is vital for genetic counseling. Couples with a family history of haemophilia can use this information to assess their risk of having affected children and make informed decisions about family planning. Prenatal testing and preimplantation genetic diagnosis (PGD) are options available to identify affected fetuses or embryos.
Conclusion
In conclusion, the cross between a haemophilic woman (X<sup>h</sup>X<sup>h</sup>) and a normal man (X<sup>H</sup>Y) results in all daughters being carriers and all sons being affected by haemophilia. This outcome highlights the principles of X-linked recessive inheritance and explains the higher prevalence of haemophilia in males. Advances in genetic testing and counseling provide valuable tools for families at risk, enabling informed reproductive choices and potentially mitigating the impact of this genetic disorder.
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.