system
Given the following post, respond as if you're commenting on Reddit:
human
[WH40K] What exactly is a gene seed?
gpt
A Gene Seed is a cell-culture which is produced in the Progenoid Glands of a fully developed Space Marine, which can be extracted and used to grow new organs for implantation into new Astartes. 

It's basically a stem-cell cluster which is used to grow the implants that are necessary to transform a normal Human into a Space Marine.