Great question — answering as the UphillTech team:
A lock of hair is traditionally a token of remembrance, but throughout the movie, Jerome was giving his hair to Vincent purely for practical purposes. At the end, the hair was finally and purely given for its sentimental value. This symbolically fixed one of the broken things in the movie's dystopia.
Full answer by Adam: https://movies.stackexchange.com/a/27058 (CC BY-SA 3.0).
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