Model Answer
0 min readIntroduction
Haemophilia is a classic example of an X-linked recessive genetic disorder, primarily affecting males due to their single X chromosome. A carrier woman possesses one normal X chromosome and one carrying the haemophilia allele, while a haemophilic man possesses the allele on his single X chromosome. Understanding the inheritance pattern of X-linked traits is crucial in genetic counseling and predicting the risk of affected offspring. This question requires us to determine the possible genetic outcomes when a carrier woman and a haemophilic man have children, utilizing the principles of Mendelian genetics and Punnett square analysis.
Understanding the Genetic Basis
Haemophilia is caused by a mutation in the genes coding for clotting factors VIII or IX. Since these genes are located on the X chromosome, the inheritance pattern is X-linked. Females (XX) have two X chromosomes, while males (XY) have one X and one Y chromosome. A carrier woman (XHXh) has one normal allele (XH) and one haemophilic allele (Xh). A haemophilic man has the haemophilic allele on his X chromosome (XhY).
Constructing the 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 |
Analyzing the Genotypes and Phenotypes
From the Punnett square, we can determine the following probabilities:
- XHXh: 50% - Carrier daughter. She has one normal allele and one haemophilic allele, so she does not exhibit the disease but can pass it on to her children.
- XhXh: 25% - Haemophilic daughter. She has two haemophilic alleles and will exhibit the disease.
- XHY: 25% - Normal son. He has one normal allele on his X chromosome and a Y chromosome, so he does not have the disease and cannot pass it on to his daughters.
- XhY: 25% - Haemophilic son. He has one haemophilic allele on his X chromosome and a Y chromosome, so he will exhibit the disease.
Detailed Explanation of Phenotypes
- Daughters: There is a 50% chance of a daughter being a carrier and a 25% chance of a daughter being affected by haemophilia.
- Sons: There is a 25% chance of a son being normal and a 25% chance of a son being affected by haemophilia.
Implications for Genetic Counseling
This analysis is crucial for genetic counseling. The couple has a 25% risk of having a haemophilic son, a 25% risk of having a carrier daughter, and a 50% risk of having a carrier daughter. The risk for each pregnancy is independent of previous pregnancies. Prenatal diagnosis, such as amniocentesis or chorionic villus sampling, can be used to determine the genotype of the foetus.
Modern Advancements in Treatment
While historically haemophilia was a life-threatening condition, advancements in treatment, such as factor replacement therapy and gene therapy, have significantly improved the quality of life and life expectancy of individuals with haemophilia. However, understanding the inheritance pattern remains vital for prevention and genetic counseling.
Conclusion
In conclusion, the cross between a carrier woman (X<sup>H</sup>X<sup>h</sup>) and a haemophilic man (X<sup>h</sup>Y) results in a 50% chance of carrier daughters, a 25% chance of haemophilic daughters, a 25% chance of normal sons, and a 25% chance of haemophilic sons. This highlights the significant risk of transmitting the haemophilia allele to both male and female offspring. Continued research into gene therapy and improved treatment options offer hope for individuals affected by this X-linked recessive disorder, but genetic counseling remains a cornerstone of managing and preventing its spread.
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.