An unrelated couple has a daughter and a son with the same rare condition. Neither parent has any obvious signs of the condition, but the father's mother (paternal grandmother) and her brother have the same condition. Select the most likely mode of inheritance:
- A. X-Linked
- B. Autosomal recessive
- C. Autosomal dominant
- D. Multifactorial
- E. Mitochondria
The correct answer is C. But I'm confused why it would be autosomal dominant when neither parent has the phenotype. I thought it might be autosomal recessive, even though this mode usually does not have a family history, just because of the fact that the paternal grandmother and her brother both had the disease, and this is consistent with the horizontal pattern associated with autosomal recessive inheritance. For it to be autosomal dominant, the disease would have to be incompletely penetrant, which seems like a less likely scenario than it being autosomal recessive.
Can someone explain why it is dominant, not recessive?