Gene Splicers are items that you can obtain through questing, achievements, or retiring a critter that has reached its max level. Gene Splicers contain traits, which can be injected into critters. You can stack injected trait levels. For example, injecting a critter with 2 Gene Splicers of the same type would give you a level 2 of that trait, rather then a level 1. Injecting traits does not cost any skill points, but due to an update, is still subject to the maximum trait level that is set by your critter's current level. Gene Splicers will not work on eggs.
The following is a list of each critter type and what gene splicer each critter will give if retired at its maximum level.