In the year 21--, a crew of 100 sets out from Earth on the good ship Tenzing Norgay, bound for Epsilon Eridani. The journey is estimated to take . . . well, quite some time, given that it was calculated that the time spent to engineer a significantly faster propulsion system (compared to the technology of the day) would negate the benefits of going at the faster speed, and would end up costing too much more.1 This means that the crew is stuck with using a ship outfitted with the Rockaway Plastopedic ThrustomaticTM 2 engines, making the trip take at least 250 years - far more than twice the lifespan of a normal human. It was also decided, to the crew's dismay, to not pursue suspended animation technology, thanks to budget cuts.
The one thing that the make of the ship did foresee was that a future generation might not be too happy about having to live and die in a ship, without ever stepping foot on a planet or doing anything besides get ready to reproduce and die. There is a very real threat of a person (or a group of people) hijacking the ship some day, and moving it to some nearby system. The computers are always on autopilot, but there's always a way to hack them.
The government decides to go with a foolproof method: using genetics to make sure that future generations on the ship will not rebel, using in vitro fertilization onboard via automatic systems. This is all very well and good, but I, the creator of the story, need to come up with a plausible explanation. I can imagine a few ideas, but they're not too good; they all boil down to neural modifications - changing genes to influence the brain. I'm no neurologist, but this doesn't sound feasible.
How can genetics be used to quell a rebellion before it begins?
1 Please, please don't dispute this. I think I can find a way to justify this, no matter what objections are made.
2 The Rockaway Plastopedic Thrustomatic is a trademark of Rockaway Blastopedic Industries, soon to be of Epsilon Eridani.