What happens at the end of Gattaca?

Spoiler: it holds. Vincent is stopped for a surprise urine test on the way to the launch pad with none of Jerome's samples on him. Dr Lamar reveals he has known for years, enters a valid result anyway, and waves him through. Vincent flies. Jerome, back at the flat, climbs into the domestic incinerator.

Lamar knew the whole time

Xander Berkeley's Lamar gives it away with a detail about how Vincent held the sample cup:

"For future reference, right-handed men don't hold it with their left."

He then talks about his own son — genetically selected, and not everything the process promised — who wants to be an astronaut. Lamar falsifies the result and sends him to the shuttle.

What happens to Jerome

Before the launch, Jerome tells Vincent he has left enough samples for two lifetimes and hands him a sealed envelope. As the rocket goes up, he puts on the silver medal from his swimming career and gets into the incinerator, leaving no body to test. In orbit, Vincent opens the envelope and finds a lock of Jerome's hair.

The other threads

• The murder — the mission director was killed by Director Josef (Gore Vidal), who wanted no chance of the Titan flight being cancelled.
• The eyelash — Vincent's stray lash at the crime scene puts Detective Hugo (Alan Arkin) and Vincent's own brother Anton (Loren Dean) on the trail of an "in-valid" inside Gattaca.
• The swim — Anton loses the open-water rematch and has to be rescued. Vincent explains he wins by never saving anything for the swim back.
• Irene — Uma Thurman's Irene Cassini learns exactly who he is before the launch, and stays.

Does Vincent come back?

The film doesn't say. The Titan mission is a year each way, and the last thing on screen is Vincent's narration as the shuttle climbs, wondering whether he's leaving or going home.

Fun fact: the title is spelled entirely from G, A, T, and C — guanine, adenine, thymine, and cytosine, the four bases of DNA.