Origin and history of trisomy

1889, from German Chromosom, coined 1888 by German anatomist Wilhelm von Waldeyer-Hartz (1836-1921), from Latinized form of Greek khrōma "color" (see chroma) + -some (3)). So called because the structures contain a substance that stains readily with basic dyes.

word-forming element of Latin and Greek origin meaning "three, having three, once every three," from Latin tres (neuter tria) or Greek trias, treis "three" (see three). "The i, etymologically short in Greek and Latin, was in Latin sometimes lengthened, esp. in numerals" [OED, 1989].

a noun suffix in words from Latin and Greek indicating state, condition, or quality (jealousy, sympathy); also activity or the result of it (victory, history); via Anglo-French and Old French -é, from Latin -ia, Greek -ia, from PIE *-a-, suffix forming abstract or collective nouns.

It is etymologically identical to -ia and the second element in -cy, -ery, -logy, etc. Many of the words were abstract in sense before concrete meanings developed (e.g. embroidery).

The suffix also is sometimes used in modern words of classical formation (inquiry), and by analogy to make alternative forms (innocence/innocency) without signification but useful metrically for an extra syllable (like Spenser's y-).