q arm of a chromosome: The long arm of a chromosome.
All human chromosomes have 2 arms -- a short arm and a long arm -- that are separated from each other only by the centromere, the point at which the chromosome is attached to the spindle during cell division. By international convention, the short arm is termed the "p arm" while the long arm of the chromosome is termed the "q arm."
Examples of usage of p and q: Loss of part of 5p (the short arm of chromosome 5) results in (Cri du Chat, the cry of the cat) the cat cry syndrome. Trisomy (an extra copy) of part of 21q (the long arm of chromosome 21) causes Down syndrome.
Origin of p and q as chromosome arms: The symbol "p" was chosen to designate the short arm because "p" stands for "petit", "small" in French. The letter "q" was selected to signify the long arm merely because "q" is the next letter in the alphabet.