The Gene-Detecting Nerve was actually part of the brain, but it was implanted in the spinal cord between the cervical and thoracic vertebrae.
It was somewhat like the implanted auxiliary brains or biological analysis devices found in other science-fiction settings. It directly connected a Space Marine's stomach lining to his brain.
The "Gene-Detecting" Nerve was designed to absorb the genetic material carrying animals' memories, experience, and innate abilities. Put simply, it was similar to Strangle's ability. Although Space Marines could not reorganize their own genes to evolve, they could read fragments of memory and survival instinct by eating an enemy's flesh.
This ability granted Space Marines extraordinary survival talents. Eating meat was learning.
For example, if a Space Marine from another Chapter arrived on Fenris with no idea where he was, all he needed to do was hunt something and eat a little of it raw.
Once the meat entered his pre-stomach, the Gene-Detecting Nerve would let him understand the habits of the planet's creatures: how to hunt, how to survive the winter, how the seasons turned throughout the year, and how to prepare to stay alive.
This was extremely useful in alien environments.
Of course, the existence of such an organ sometimes led to unconventional uses that seemed somewhat heretical.
Some Space Marines secretly consumed the corpses of their enemies to learn the weaknesses, habits, instincts, and other traits of enemy species, even developing further uses for it.
One such force was the Blood Angels successor Chapter, the Blood Drinkers. Their Gene-Detecting Nerves had become highly mutated, allowing them to suppress the onset of the Black Rage by drinking human blood.
The Black Rage was a hereditary disease of the Gene-Seed, chiefly caused by the Blood Angels' Primarch, Sanguinius. He had died in the final battle against the Traitor