MCAT® ExamBiological and Biochemical Foundations of Living SystemsHard
A geneticist is studying a family pedigree where a rare genetic disorder appears to affect only males, skipping generations, and is transmitted from an affected grandfather to his grandsons through their unaffected mother. This inheritance pattern is most consistent with which of the following?
- AX-linked recessive
- BX-linked dominant
- CAutosomal dominant
- DAutosomal recessive
Show answer & explanationAnswer & explanation
Correct answer: A. X-linked recessive
The description 'affects only males,' 'skipping generations,' and 'transmitted from an affected grandfather to his grandsons through their unaffected mother' are classic hallmarks of X-linked recessive inheritance. Males are more frequently affected because they only have one X chromosome, and mothers are often carriers (unaffected) who pass the trait to their sons.
Why the other options are wrong
- B. X-linked dominant affects both sexes, but more females, does not skip generations, and affected fathers pass to all daughters.
- C. Autosomal dominant affects both sexes equally, does not skip generations, and is passed directly from affected parent to child.
- D. Autosomal recessive affects both sexes equally, skips generations, but male-to-male transmission is possible, and the mother acting as an unaffected carrier for male offspring is less specific than X-linked.
X-linked Recessive Inheritance
A pattern of inheritance where a recessive gene on the X chromosome causes a trait to be expressed more frequently in males than females, often skipping generations, and is transmitted from carrier mothers to their sons.
- More common in males than females.
- Affected sons usually born to unaffected mothers (carriers).
- Never passed from father to son.
- Affected fathers pass the allele to all daughters, making them carriers.
- Often skips generations.
Memory trick: X-Recessive X-Files: Males are the mystery, mothers are the secret agents, fathers never transmit to sons!