User blog comment:Godisme/Everything But The Rain/@comment-1747976-20130304102648/@comment-1393830-20130306145022

Its exactly that. The Zanpakuto is born from the soul. They have similar Zanpakutos because they are father and son. Ichigo did not have to inherit Shinigami powers from his father in order to have a similar Zanpakuto, all he needed was to be similar to his father, which he is. A Zanpakuto is born from the Soul and personality of its owner, because Isshin and Ichigo are similar personality wise and because they are father and son, it stands to reason that their Zanpakuto would be similar.