Genies refers to genetically modified humans. Genetech engineering of the human germ cell line became well established by the 22C. The major motivation was to increase the range of conditions that humans could survive in. Some were done for improve military performance, Neural interfacing performs better in those with the NAPP23 mod. More extreme mods ('tweaks') were performed to allow survival in even more extreme hostile environments.
Over the centuries, many of these gene mods have spread, largely due to the human tendency to exchange bodily fluids, especially with that which is novel, strange or forbidden. While many in the Prime movement emphasizes the importance of human genetic purity, studies have shown that even on the oldest and most isolated prime worlds, each individual has between 3 and 5 modded genes. So nowadays, we are all genies. 
The term tweak is generally resevered for a human who has been radically genetically modified for environmental or aptitude purposes. The word tweaks is regarded by many as derogatory, not the least because it rhymes with freaks. Tweaking featured early in human interstellar exploration, enabling colonization of some of the more hostile exoplanets, the most famous of which is Hades.  
Tweaks is also used to designate any Terran organism who has been so radically modified for environmental or aptitude purposes. Some animals have been tweaked to provide a food or resource that can survive on exoplanets, others as companion animals.