"As with other things, such as smoking"

There may not be a genetic link, however people who's mothers smoked, drank or did drugs while they were pregnant are much more likely to form addictions when they grow up.

It has to do with production of Dopamine and the abnormalities caused in Dompamine production as a result of addictive chemicals affecting the formation of the brain in the womb.

There are thousands of pages to sift through on the web so I'll leave you to it.