Physiology & Biochemistry · patient case question
His father is unaffected and his mother is a carrier. What is the probability that his SISTER is PHENOTYPICALLY AFFECTED?
Patient case: Adolescent with Duchenne muscular dystrophy
Patient: Male, 15 years old, in a wheelchair
Chief complaint: “My back tooth is broken and it hurts.”
History: Duchenne muscular dystrophy diagnosed at age 4. His father is unaffected and his mother is a known carrier. He has one sister.
Findings: Non-restorable tooth #19. Limited mouth opening. Macroglossia is noted. Poor oral hygiene owing to reduced upper limb strength.
Show answer and explanation
Correct answer: A. 0%
- A. CORRECT. Duchenne muscular dystrophy is X-LINKED RECESSIVE. The daughter receives one X from her father and one from her mother. Her father is UNAFFECTED, so the X he transmits carries a normal allele, and that normal allele is dominant over the mutant one. She therefore CANNOT be phenotypically affected, whatever she inherits from her mother. Her risk of being a CARRIER is 50 percent, which is the number the question is baiting you toward.
- B. 100 percent would only apply if both her X chromosomes necessarily carried the mutation — impossible with an unaffected father.
- C. 25 percent would be the affected-offspring figure for an AUTOSOMAL RECESSIVE condition with two carrier parents. That is a different inheritance pattern.
- D. 50 percent is the probability that she is a CARRIER, not that she is affected. Confusing carrier status with affected status is the single commonest error on X-linked pedigree questions.
Note: The full cross — carrier mother XᴿXʳ by unaffected father XᴿY: sons are 50% affected, daughters are 50% carriers and 0% affected. A small minority of female carriers show mild symptoms through skewed X-inactivation, but that is not what 'phenotypically affected' means on an exam.
Practice Physiology & Biochemistry questions Physiology & Biochemistry study notes