User blog comment:StonerCatJay/Rereading the series- theories and speculation/@comment-3230060-20140522160420/@comment-5551986-20140523174201

Isshin did use Getsuga Tenshou in his fight with White, he just didn't use it on White's physical body but rather his Hollow Soul or Hollow powers corrupting Masaki, http://www.mangapanda.com/bleach/536/3. Dunno about you guys, but his Soul and Shinigami powers were connect to Masaki, but using it after such a short time? I don't think so. And seeing as how Ichigo's Inner Yhwach only used it in fillers is quite contradictory as to whether its a Quincy ability. The craziest theory I have is that its a sword technique and not a Zanpakuto ability, maybe passed down through the Shiba clan, after all they like Fireworks and a Getsuga Tenshou is pretty flashy ;) But I doubt even that, I'd go ahead and say my most logical thought it that, either children partially inherit some of their parents souls, giving them "similar" Zanpakuto if they have spiritual powers. Or that Ichigo being born a quincy absorbed reishi and spiritual power in his mother's womb almost feeding similarly to a fetus and ending up with a part of Isshin's power and Whites, which were conflicting and suppressed in Masaki's soul.