Wax yar ka dib, cytologists (cilmi-nafsiga unugyada) ayaa soo jeediyay in chromosomes ay ahaayeen maaddada hiddaha. Inta u dhaxaysa 1910 iyo 1915, Thomas Hunt Morgan iyo "Drosophilists" ee shaybaarkiisa duulaya waxay ku dhajiyeen labadan fikradood - labadaba muranka ka dhashay - "Mendelian-chromosome theory" ee hiddaha. Waxay ku qiyaaseen ifafaalaha hiddaha waxayna soo jeediyeen in hiddaha ay ku jiraan chromosomes sida beads; waxay ku tilmaameen inay sharraxayaan iskutallaabta hiddaha ee loo yaqaan 'overage' iyo dhismaha maaster melanocytes ee duulaya, taas oo noqotay qaab loo adeegsaday Mendel's (oo aan dhab ahaantii ku jirin shaqada Mendel). Wax yar ka dib, cilmi-nafsiga unugyada (cilmi-nafsiga unugyada) ayaa soo jeediyay in chromosomes ay yihiin maaddada dhaxalka ah. Intii lagu guda jiray 1920-kii, Fisher Hunt Morgan iyo "Drosophilists" ee shaybaarkiisa du
unugyada