'chromosomally' originates from Modern English, specifically the adjective 'chromosomal' + the adverbial suffix '-ly', where 'chromosomal' comes from 'chromosome' (see below).

'chromosome' was coined in the late 19th century (German 'Chromosom', from Neo-Latin 'chromosoma') from Greek 'chroma' meaning 'color' and 'soma' meaning 'body'; this became English 'chromosome', then adjective 'chromosomal', and finally adverb 'chromosomally' by addition of '-al' and '-ly'.

Initially 'chromosome' literally meant 'colored body' (referring to stainable bodies in cells); over time it evolved to mean the DNA-containing structures recognized as chromosomes, and 'chromosomally' now means 'in a way that concerns chromosomes'.