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?

Verified step by step guidance

1. span>Step 1: Understand the inheritance pattern of hemophilia A, which is an X-linked recessive condition. This means the gene causing the condition is located on the X chromosome, and two copies of the gene (one from each parent) are needed for females to express the condition, while males need only one copy (from their mother) to express it.</span
2. span>Step 2: Analyze the family history. S.R.'s maternal grandfather had hemophilia A, indicating he had the affected X chromosome. S.R.'s maternal grandmother did not have the condition, so she must have had at least one normal X chromosome.</span
3. span>Step 3: Determine the genetic status of S.R.'s mother. Since S.R.'s maternal grandfather had hemophilia A, S.R.'s mother must have inherited the affected X chromosome from him, making her a carrier (X^HX).</span
4. span>Step 4: Assess S.R.'s genetic status. S.R. has a 50% chance of inheriting the affected X chromosome from her mother, making her a carrier (X^HX), and a 50% chance of inheriting the normal X chromosome, making her not a carrier (XX).</span
5. span>Step 5: Calculate the probability of S.R. having a son with hemophilia A. If S.R. is a carrier, there is a 50% chance she will pass the affected X chromosome to her son, who will then have hemophilia A. If S.R. is not a carrier, the probability is 0%. Therefore, the overall probability is the chance of S.R. being a carrier multiplied by the chance of passing the affected X chromosome to her son.</span

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.