A researcher at the University of Amsterdam has concluded that synaesthesia might not be merely genetic:
To test the idea, they gave seven volunteers a novel to read in which certain letters were always written in red, green, blue or orange (see picture). Before and after reading the book, the volunteers took a “synaesthetic crowding” test, in which they identified the middle letter of a grid of black letters which were quickly flashed onto a screen. Synaesthetes perform better on the test when a letter they experience in colour is the target letter.
The volunteers performed significantly better on this test after training compared with people who read the novel in black and white.
Seven is a really small sample size. This needs to be reproduced with larger samples to be accepted.