Diane,
No one can say for sure about MM being genetic, however the cases I read about were from a parent with MM then their children having MM.
My son is the first on in the family to have a stroke, so I can't even say their is a history of strokes that might have been undiagnosed MM.
By my understanding then, my son has a higher chance of having children with MM, but my daughters chances are as low as the rest of the population.
Hope I didn't confuse you more. 
Anyone have more info? 
Rena

hi my name is Sandy i have been told that i have the dominate gene for moyamoya there is a lot of test beimg done on my kids and i because all of my kids have mm i have 4 kids and 3 of them have mm the Dtr. said that they still have not pin pointed the gene

DJ, thanks for that info.It sure is interesting as well as promising.I hope this starts alittle more research in that direction.I have 3 children and watch my ohter 2 closely.It has always seemed so much more than just coincidence to see families with more than one child with such a rare disease.Also who knows what is in our backgrounds this has been miss diagnosed for so long.I have a huge history of strokes,"spells" as they were referred toback then, who knows what it was , Migraines, and Circle of Willis Aneurisms.It just seems tomuch of a coincidence.In fact my daughters first stroke was on the 20 yr anniversary of my fathers death from a cirlce of Willis aneurism,and the death of his father from a stroke.Sure was "creepy" .Thanks again for allthe info you supply here.
Mary Grace