alt Hacker NewsGenerics is always claimed to be a reason, but nutrition also runs in families.
It's like saying BRCA1 causes breast cancer, but 90% of cases don't involve the gene, and many people with the gene don't get cancer.
I'm not saying it can't be a reason, but it looks like it's a common and maybe convenient excuse.
There is an easy line to draw between the two. "Family history", which includes eating/smoking/drinking habits, follows habitation. Genetics follows blood. So differences between bloodlines easily stand apart from family trends.