Using hair/eyelashes for genetic testing will fail if no follicle is attached, but this method is used when no other material is available or the information is being obtained surreptitiously (as Irene and the investigators are doing when they use this method). Urine is not the most efficient way to perform genetic testing when other samples are readily available, but the urine test was also performed to look for drug use as evidenced by Eugene's sample failing when he was drunk. More specifically, urine itself does not contain DNA, but it may contain epithelial cells, which do contain DNA. Most healthy individuals, however, do not excrete epithelial cells in their urine. http://www.biology.arizona.edu/human_bio/problem_sets/DNA_forensics_2/06t.html
In the beginning when Vincent is putting the blood into the fake thumb he puts it on his left thumb but he always uses his right hand to get tested. This is because Vincent himself is left-handed, but Jerome is right-handed so he had to make the switch when he assumed Jerome's life.
How did Irene cross the highway that easily? Isn't that extremely dangerous?
A
It's indeed mortally dangerous for a normal human being like VIncent even to attempt such a reckless act. However, Irene was genetically designed like most (if not all) of the drivers in the highway, so it could be argued that most of the Valid (genetically enhanced individuals) have far better senses and reflexes other than the InValid or God's Children (normal human beings), and an inherent lowel level of fear in front of dangerous situations. Note how Gattaca's world seems to have no traffic light whatsoever, for instance. Maybe they are not needed at all.
Q
Is in fact that song only playable if you have twelve fingers?
A
The piece played by the six-fingered pianist (http://www.youtube.com/watch?v=w5FowPApEzY) is a variation on Impromptu in G Flat Major, D899, No. 3. by Franz Schubert (1797-1828). The original version is obviously possible to play at 10 fingers (for instance, watch this version: http://www.youtube.com/watch?v=L6_SbflSwAg), however, Michael Nyman (who composed the entire soundtrack and made this arrangement) did a beautiful job embellishing the piece with additional notes/harmonies to something that "can only be played with twelve" fingers, as the Irene Cassini (Uma Thurman character) notes. The piece was digitally composed from the human-capable pieces put together with the impossible to play ones.Many experienced pianists have noted how this variation is in fact impossible to play by human beings, not only because the lack of additional fingers, but also the uncomfortable way it would be to attempt it as a duet.