The genetic splicer can be found in both Creatures 2 and Creatures 3. On both versions you load two different creatures (Norn, Ettin or Grendel) into the machine and press a button or switch. The two creatures' dDNA is then cut apart and mixed together to create a single new creature. The original two creatures are killed in the process.
There is, however, a glitch in Creatures 2 which allows you to save one of the victims of your genetic experiments. If you watch the cages carefully after starting the splicing process, you will have a short time after the purple flashes appear to press the button on one of the cages. Timing is critical, but if you press the button quickly enough after the flashes start, the flashes in that cage will freeze and the creature in the cage will be saved while still becoming part of the resulting spliced creature. If this method goes wrong, however, you can end up wit the purple flashes stuck, the creature unable to get out. If the splicer is re-run, it will create an egg that will never hatch.
An alternate, safer method to prevent the death of spliced creatures is to use the SafeSpliceCOB, for which a guide can be found here.
In both games the splicer must be activated by pressing switches or collecting power-ups.
In Creatues 2, creatures that are old or dying may be saved by putting them in the splicer on their own. The original will be destroyed, and a fresh egg containing a clone of the creature will be produced.
In Creatures 2, when the family tree of the new creature is examined, you will notice that the parents are the two creatures that you combined. It will also say mother and father on the birth certificate, regardless of genders of the original creatures (currently not tested in Creatures 3).