User blog comment:Godisme/Everything But The Rain/@comment-1747976-20130301204345/@comment-4431351-20130302001954

Well, actually you can Godisme. Imagine you being colorblind, while both of your parents can see colors perfectly. It means your mother is a carrier of the recessive gen. While she herself can see colors with no problem, you can not, as you inherited the recessive gene. Now reflect this on Ichigo. We know from the Fullbring Arc, that losing your Shinigami powers doesn't mean losing all your reiatsu. Ichigo still had a little speck of reiatsu deep within himself. This little speck was triggered by the other Shinigami's reiatsu (Rukia with the reiatsu sword) and resurrected his own powers.

Why can't it been the same with Isshin? A little speck remained and was over a period of 20 years resurrected, resulting in him regaining his Shinigami powers. He still had the "gene" inside him. Isshin could have given this "gene" through to his children, giving them the possibility to become Shinigami when the "gene" is awakened by a stimulation of reiatsu (Rukia's and Ichigo's first meeting).

I have also reread two different translations of chapter 188. The wording is kinda ambiguous. One says "being back in a Shinigami's body after 20 years" the other "to take the form of a Shinigami that you abandoned 20 years ago". None of them really says he lost his powers. He just could have not used them in a 20 years, maybe to life a peaceful life, stay undetected and so on. However chapter 409 does insinuate Isshin has once used FGT, resulting in him losing his powers, just like Ichigo a couple of chapters later. So not closing out my previous deduction.