A 34-year-old, G1P0, presents for genetic counseling at 12 week’s gestation. The patient has two sisters and a brother; her father has hemophilia. Her siblings are not affected, but she has a nephew that is:What is the inheritance pattern of this disorder?
**Core Concept**
The question is testing the student's understanding of the inheritance pattern of a genetic disorder, specifically hemophilia. Hemophilia is an X-linked recessive disorder, meaning it is caused by mutations in genes on the X chromosome. Males have one X and one Y chromosome, while females have two X chromosomes.
**Why the Correct Answer is Right**
The correct answer is due to the fact that hemophilia is an X-linked recessive disorder. The patient's father has hemophilia, indicating that he is likely affected because hemophilia is more common in males. The patient's two sisters are not affected, suggesting they are carriers of the mutation. However, the patient's nephew is affected, meaning he must have inherited the mutated gene from his mother, who is likely a carrier. This is possible because females can be carriers of X-linked recessive disorders, even if they are not affected themselves.
**Why Each Wrong Option is Incorrect**
* **Option A:** This option would imply that hemophilia is an autosomal dominant disorder, which is incorrect because hemophilia is not inherited in an autosomal dominant pattern.
* **Option B:** This option would imply that hemophilia is an autosomal recessive disorder, which is incorrect because hemophilia is not inherited in an autosomal recessive pattern.
* **Option D:** This option would imply that hemophilia is an X-linked dominant disorder, which is incorrect because hemophilia is not inherited in an X-linked dominant pattern.
**Clinical Pearl / High-Yield Fact**
It's essential to remember that females can be carriers of X-linked recessive disorders, even if they are not affected themselves. This is because they have two X chromosomes, and a single copy of the mutated gene is enough to cause the disorder in males.
**Correct Answer:** C. X-linked recessive inheritance.