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 muran badan - "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 ka soo gudubka iyo dhismaha jilicsanaanta hiddaha ee duuliyaha, taas oo noqotay qaab aad u wanaagsan oo loo adeegsaday si ay u soo saaraan hiddaha.
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