User blog comment:Godisme/Everything But The Rain/@comment-1933234-20130227175549/@comment-1351982-20130227205948

At the time Ichigo was conceived, Isshin was human. Ichigo is just a human/quincy hybrid, not a shinigami/quincy hybrid. In order for Ichigo to be a true shinigami/quincy hybrid, Isshin would have needed to be in soul form at the time he and Misaki conceived Ichigo, which he was not.