Comprehensive and Detailed Explanation:

In X-linked recessive disorders such as hemophilia A, carrier mothers (usually asymptomatic) have a 50% chance of passing the affected X chromosome to each son, who would then express the disease. Each daughter has a 50% chance of being a carrier but is generally not affected.

Toronto Notes 2023 – Genetics:

“X-linked recessive inheritance: Carrier mother has a 50% chance of having an affected son and a 50% chance of having a carrier daughter.”

MCCQE1 Objectives (Genetics > 61-1: Inheritance Patterns):

“Candidates must apply principles of X-linked inheritance to assess risk in offspring.”

If the mother is a known carrier (as inferred from having an affected son), the chance of a second affected boy is 50%.

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