Physiology & Biochemistry · patient case question
Her mother is a carrier of Duchenne muscular dystrophy and her father is unaffected. What is the probability that any given daughter of that couple is a carrier?
Patient case: Family History and a Preoperative Workup
Patient: 26-year-old female
Chief Complaint: “My wisdom teeth need to come out and I want to make sure everything is safe first.”
Background: Multiple sclerosis diagnosed 2 years ago, currently in remission. Total thyroidectomy 6 months ago for papillary thyroid carcinoma, with persistent hoarseness since surgery. Her mother is a known carrier of Duchenne muscular dystrophy. Her brother has hemophilia A. She is planning a pregnancy within the next year.
Medications: Levothyroxine · Glatiramer acetate
Vital Signs: BP 114/70 mmHg · HR 76 bpm · RR 16 · T 98.4°F
Findings: Persistently breathy, hoarse voice. Well-healed anterior neck incision. Four impacted third molars planned for removal. Ibuprofen is planned for postoperative analgesia.
Show answer and explanation
Correct answer: A. 50%
- A. Correct. Duchenne muscular dystrophy is X-linked recessive. A carrier mother has one affected and one normal X chromosome, and each daughter inherits one of them at random along with a normal X from her father. Each daughter therefore has a 50% probability of being a carrier.
- B. Incorrect. Twenty-five percent is the probability that any given child — counting both sexes — will be a carrier daughter, since it combines a 50% chance of being female with a 50% chance of inheriting the affected X. Read carefully for whether the question specifies a daughter or a child.
- C. Incorrect. All daughters would be carriers only if the father were affected, since he would pass his single affected X to every daughter.
- D. Incorrect. Seventy-five percent does not correspond to any outcome in this cross.
Note: Full cross for a carrier mother and unaffected father: 25% affected son, 25% unaffected son, 25% carrier daughter, 25% unaffected daughter.
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