They've been hinting at a strong 'supernatural genetics' thing the entire series. Hexenbiests have been the most prominent example of this, though, specifically the moment in S1 when Adalind's powers went woosh out of her like an exorcism.If your remember back near the beginning of the series they also tried to explain Wessin/Vessen with the theory of "chimeric DNA", and then completely dropped and ignored that angle.