In the monster romp “Splice,” biochemists Clive (Adrien Brody) and Elsa (Sarah Polley) use DNA splicing to hatch a half-human, half-animal creature in their lab, then take a parental shine.

Big mistake. Scientists, whether sane or mad, should never treat their lab subjects like offspring. Disaster is inevitable.

But Clive and Elsa have obviously never watched “Frankenstein,” and the movie none-too-subtly lets us know they’re thinking about starting a family. Why not try things out with this deformed, mewling slug? The horrible-yet-strangely-cute mutant grows up quickly (she ages days within minutes) and becomes Dren (Delphine Cheneac), a humanoid with peculiar characteristics, such as vaguely supermodel looks, wings that sprout when she becomes angry or aroused and a tail with a phallic stinger that pumps poison.

Dren is usually gentle — she likes kittens, sugary foods and playing with toys — except when she throws a tantrum, in which case tranquilizer darts and a titanium cage might come in handy. But Clive and Elsa, who have squirreled Dren out of their laboratory and to a remote farmhouse where they can raise her away from their meddling bosses, haven’t really prepared for the worst-case scenario.

“Splice” was directed by Vincenzo Natali, who made the fiendishly creepy, low-budget shocker “Cube,” and he fares well in the scenes depicting a pre-grown Dren and her brethren — fleshy slugs you don’t know whether to pet or euthanize.

But although Brody and Polley do their best to sell the increasingly illogical behavior of their characters, “Splice” grows progressively sillier the older Dren gets, finally resorting to sudden “gotcha!” appearances by a pissed-off mutant, who is none too happy about the corrective behavior her parents have planned for her. The creature effects are terrific, but the human protagonists aren’t nearly so interesting.

By the time the film’s climactic 15 minutes rolled around, viewers at a preview were laughing as if they were watching “Knocked Up.” For a horror picture, such a reaction is the equivalent of a stake through the heart.