p arm of a chromosome: The short arm of a chromosome. The "p" comes from the French "petit" meaning small. All human chromosomes have 2 arms - the p (short) arm and the q (long) arm - that are separated from each other only by a primary constriction, the centromere, the point at which the chromosome is attached to the spindle during cell division. The symbol "q" was chosen for the long arm simply because it followed "p" in the alphabet.
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