As noted by Vuneca on official forums, when you have The Sims 3 Seasons & University Expansions installed, the Forbidden Fruit offers a new way to create and add an Alien to your household.
Aliens who “pick” the forbidden fruit have a chance of receiving an “Alien Plant Baby”. This type of PlantSim baby will not acquire the PlantSim occult trait.
Aliens who are successful at picking a Plant Baby will pass on 50% of their DNA bloodline. Any DNA percentage lower than 20%, is no longer considered an Alien in the game, and therefore has no alien powers. Babies who have less than 20% of Alien DNA will acquire the PlantSim powers.
For more information on PlantSims, please refer to our University Game Guide
❗ These are DNA samples from a test I personally conducted, so results will vary in different games
♦ 100% Alien DNA + Forbidden Fruit Baby Pick = 51.8% (+/- 10%) Alien DNA
♦ 50% Alien DNA + Forbidden Fruit Baby Pick = 21.3% (+/- 10%) Alien DNA
♦ 21.3 Alien DNA + Forbidden Fruit Baby Pick = 15.2% (+/- 10%) Alien DNA (No Alien Powers; Is PlantSim)