Layer Cake -OR- That Self-Indulgent SPN Omegaverse I'm Writing
John and Mary Winchester were a very special couple. John was in the Vessel Line of the Archangel Michael, Mary the Line of Lucifer himself. All the forces of Heaven had concentrated on getting them together, and keeping them so until they had a few kids. Mary was born a hunter, it was her Family Business. John was a Man of Letters by legacy, even if he didn’t know it.
But there was also another way in which they were special, a much more mundane way. John was an Alpha, with the genotype A/o. Mary was a Beta, with the genotype B/o. Unlike most couples, they were genetically capable of producing offspring of any Tier.
This was only made evident approximately thirteen years after the birth of their eldest child.
The One in Which Dean goes into Pre-Heat, no one knows what's going on, and everyone freaks out.