A woman, S.R., had a maternal grandfather with hemophilia A (OMIM 306700), an X-linked recessive condition that reduces blood clotting. S.R.'s maternal grandmother and paternal grandparents are free of the condition, as are her partner, his parents, and his grandparents. S.R. has no siblings. She wants to know the chance that a son of hers will have the condition. What is that probability?

Key Concepts

X-Linked Recessive Inheritance

Hemophilia A is an X-linked recessive disorder, meaning the gene responsible for the condition is located on the X chromosome. Males, having one X and one Y chromosome, are more likely to express the condition if they inherit the affected X. Females, with two X chromosomes, must inherit two copies of the mutated gene to express the disorder, making them carriers if they have only one affected X.

Carrier Status

In this scenario, S.R. is the daughter of a man with hemophilia A, which means her father contributed an affected X chromosome. However, since her mother is not affected and has a father with hemophilia, S.R. is likely a carrier of the hemophilia gene. Understanding carrier status is crucial, as it determines the probability of passing the affected gene to her offspring.

Probability of Inheritance

To calculate the probability of S.R.'s son inheriting hemophilia A, we consider that a son inherits his X chromosome from his mother and a Y chromosome from his father. If S.R. is a carrier, there is a 50% chance she will pass on the affected X chromosome to her son. Thus, the probability of her son having hemophilia A is directly linked to her carrier status and the inheritance pattern of X-linked traits.