Genetics is undoubtedly a factor - but the fact is that of all those who have used marijuana, 9% have at some point been dependent, whereas the corresponding figure for alcohol is a substantially higher 15% (and 32% for tobacco).
the problem I see is what happened to Karen Quinlan in 1975 when she attended a party:
“She had eaten almost nothing for two days. At the party she reportedly drank a few gin and tonics and took Quaaludes. Shortly afterwards she felt faint and was quickly taken home and put to bed. When friends checked on her about 15 minutes later, they found she was not breathing.”
My point is when a person tries anything, cigs, booze, weed or any of the new designer drugs, you have no idea if one of your genetic time bombs might go off. Besides that, whenever they show videos of weed users in Colorado, I am not impressed.