A sonomic chromosome was any chromosome that was not a sex chromosome.

In 2367, while performing a DNA analysis on Ishara Yar to determine if she were Natasha Yar's sister, Doctor Beverly Crusher told her there was always differentiation in individuals' sonomic chromosomes, but this would not affect the results. (TNG: "Legacy")

The script referred to this by the correct term somatic chromosome, but Gates McFadden stated sonomic chromosome in the episode. [1]