Alison's birth certificate lists her last name as Hendrix. But her husband Donnie's last name is also Hendrix in this wiki. At least one of the following must be true:
1. We were never told Donnie's last name. Some fans just assume it's Hendrix, when it is not.
2. Donnie's last name is coincidentally also Hendrix, but he and Alison are not related.
3. Donnie and Alison share the same last name because they are brother and sister... that's the main reason they have trouble producing offspring of their own, and not all this clone engineering hocus-pocus.
That would be consistent with his weenie personality (at least as portrayed thus far). Also, husbands taking the wife's name is a common occurence in Japan when the continuance of her family name is in jeopardy.