Grabiya's Theorem

Grabiya's Theorem was the posited solution put forth by Terran gene-wrights during the Great Crusade era to solve the diminishing stability of the Gene-Seed stores through over-use, due to the continued expansion of the nascent Imperium by the Emperor's Space Marine Legions. This theorem demonstrated that a Primarch's genetic code could be used to stabilise and expand existing Astartes gene-seed stocks with what was hoped to be "minimal deviation."

History
As the Great Crusade continued the expansion of the nascent Imperium into the galaxy, the discovery of the Primarchs and their newly adopted homeworlds helped to stem an impending crisis that was not widely known of at the time outside of the exalted ranks of the Imperium's ruling War Council: namely, the diminishing stability of the gene-seed itself through over-use and the increasing need for ever greater numbers of Space Marines in the field. This was a matter that only worsened as the Great Crusade pushed ever wider afield into the galaxy. Imperial forces could no longer be concentrated as easily as before, and attrition was taking its toll as years of near-constant battle became decades. To relent the pace of the Great Crusade's progress was for the Emperor simply not an option and so the simple truth was that more Space Marines were needed and they needed to be created faster than before.

A secret conclave of gene-wrights under the Emperor's direct supervision posited the solution that became known as Grabiya's Theorem, which demonstrated that a Primarch's genetic code could be used to stabilise and expand Astartes gene-seed stocks with what was hoped to be "minimal deviation." Alongside this accelerated gene-culturing technique, other previously unavailable genetic technologies were put into effect, reducing the processing time required to create a battle-worthy Space Marine to a single Terran year in some cases. Such accelerated gene-seed techniques, along with absent, inadequate or over-forceful psycho-doctrination techniques, were later found to have unseen fundamental flaws. Many Imperial savants since have come to believe that the drive to create larger Space Marine Legions at accelerated speed played a prime role in the degradation of the sanity and psychological make-up of certain Legions and paved the way for the horror of the Horus Heresy that came later.