Board Thread:General Discussion/@comment-33053086-20171006004010/@comment-25396609-20171122001535

I saw a theory ages ago that it was parasitic, based the summoning being something they've been able to do "for generations".

The interpretation of it being parasitic is that rather than being n original part of the semblance it was something "acquired" from someone who married into the bloodline, with it being added into the semblance their children inherited.

tl;dr

Person with Semblance A has kids with a Schnee who has Semblance B

Schnee child  is born with Semblance B that has now acquired the effects of Semblance A

A bit convoluted, but it was a theory that existed at some point.