MCAT® ExamBiological and Biochemical Foundations of Living SystemsMedium
A geneticist is studying a family pedigree where a rare genetic disorder shows the following pattern: Affected individuals appear in every generation, affected fathers transmit the trait to all their daughters but none of their sons, and affected mothers transmit the trait to approximately half of their children, regardless of sex. What is the most likely mode of inheritance for this disorder?
- AX-linked recessive
- BAutosomal recessive
- CAutosomal dominant
- DX-linked dominant
Show answer & explanationAnswer & explanation
Correct answer: D. X-linked dominant
The pattern of affected fathers transmitting to all daughters but no sons is a hallmark of X-linked dominant inheritance. Affected mothers transmitting to half their offspring is also consistent, as they have two X chromosomes.
Why the other options are wrong
- A. X-linked recessive would show affected males more frequently and affected fathers would not transmit to all daughters.
- B. Autosomal recessive typically skips generations and affects males and females equally.
- C. Autosomal dominant would affect sons and daughters equally, and fathers would transmit to both sexes.
X-linked Dominant Inheritance
A mode of inheritance where a dominant gene on the X chromosome causes the trait. Affected fathers pass it to all daughters.
- Affected individuals in every generation.
- Affected fathers pass to ALL daughters, NO sons.
- Affected mothers pass to 50% of children (both sexes).
- More common in females than males (but males often more severely affected).
Memory trick: X-Dominant: Dad Daughter Direct