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 of their daughters but none of their sons, and affected mothers transmit the trait to half of their sons and half of their daughters. What is the most likely mode of inheritance for this disorder?
- AX-linked dominant
- BAutosomal dominant
- CX-linked recessive
- DAutosomal recessive
Show answer & explanationAnswer & explanation
Correct answer: A. X-linked dominant
The key characteristics point to X-linked dominant inheritance: 1) affected individuals in every generation (dominant), 2) affected fathers transmit to all daughters (because daughters always inherit their father's X chromosome) but no sons (sons inherit Y from father), and 3) affected mothers transmit to half of sons and half of daughters (each offspring has a 50% chance of inheriting the affected X chromosome from the mother). Autosomal dominant would affect sons and daughters equally from an affected father. X-linked recessive would typically show affected males more often and affected fathers would not transmit to all daughters.
Why the other options are wrong
- B. Autosomal dominant disorders appear in every generation and affect males and females equally; affected fathers would transmit to half sons and half daughters, not all daughters.
- C. X-linked recessive disorders typically affect males more, and affected fathers cannot pass the trait to sons; affected mothers pass to half sons.
- D. Autosomal recessive disorders skip generations and affect males and females equally; affected parents may have unaffected children.
X-linked Dominant Inheritance
X-linked dominant inheritance occurs when a dominant gene on the X chromosome causes a trait to be expressed. Affected individuals typically appear in every generation, and characteristic patterns of transmission occur between parents and offspring.
- Affected individuals in every generation
- Affected fathers pass to ALL daughters, NO sons
- Affected mothers pass to 50% of sons and 50% of daughters
- Males and females can be affected, but females may be more mildly affected due to X-inactivation
Memory trick: X-Dominant: Dad's X is Daughter's Destiny.