A person (let's just call him "John") had 3 aunts that died back in the 1950's in early childhood due to an unknown form of neuromuscular disorder. One died at birth, one at 2 years and one at 3 years. John's father was the only surviving descendent of John's grandparents. John and his brother do not show any symptoms. John's brother has a daughter who shows no symptoms. John has two sons, one does not show any symptoms and his other son is a 4 month old who is exhibiting signs of some sort of neuromuscular problem.
John has provided me with a pedigree that he believes is correct (note, the ones with the blue square are the ones that exhibit the symptoms):
I took a bit of genetics in university, but basically I am just trying to understand the genetics here. These are my assumptions. Please correct me if any are wrong:
- Since the disorder appears to prevent developing past childhood it must be autosomal recessive because everyone who exhibits symptoms is not be able to reproduce due to their early death.
- Both of John's grandparents must have been carriers.
- The probability of John's dad and any aunts and uncles having the disorder is 25%, and the probability of being a carrier is 50%. The 3 aunts who died must have just lucked out.
- John, his dad, and his wife must all be carriers of the disorder.
However I really feel like I'm wrong about these assumptions since the math really doesn't add up. The likelihood that so many carriers for a relatively rare disorder all coming together and the fact that 3 of the 4 offspring of the grandparents died with only a 25% chance all seems improbable.
It could be that John's son doesn't have the disorder and what he has is something completely different. What do the geneticists say?