▁occurs ▁by ▁the ▁X : A ▁ratio ▁of ▁X ▁chrom os om es ▁to ▁aut os om es , ▁not ▁because ▁of ▁the ▁presence ▁of ▁a ▁Y ▁chrom os ome ▁as ▁in ▁human ▁sex ▁determ ination . ▁Although ▁the ▁Y ▁chrom os ome ▁is ▁entirely ▁heter och rom atic , ▁it ▁contains ▁at ▁least ▁ 1 6 ▁gen es , ▁many ▁of ▁which ▁are ▁thought ▁to ▁have ▁male - related ▁functions . ▁ ▁Similar ity ▁to ▁humans ▁ ▁A ▁March ▁ 2 0 0 0 ▁study ▁by ▁National ▁Human ▁Gen ome ▁Research ▁Institute ▁comparing ▁the ▁fruit ▁fly ▁and ▁human ▁gen ome ▁estimated ▁that ▁about ▁ 6 0 % ▁of ▁gen es ▁are ▁conser ved ▁between ▁the ▁two ▁species . ▁About ▁ 7 5 % ▁of ▁known ▁human ▁disease ▁gen es ▁have ▁a ▁recogn izable ▁match ▁in ▁the ▁gen ome ▁of ▁fruit ▁f lies , ▁and ▁ 5 0 % ▁of ▁fly ▁protein ▁sequences ▁have ▁m amm al ian ▁hom olog s ▁. ▁An ▁online ▁database ▁called ▁Hom oph ila ▁is ▁available ▁to ▁search ▁for ▁human ▁disease ▁gene ▁hom olog ues ▁in ▁f lies ▁and ▁vice ▁vers a . ▁ ▁D ros oph ila ▁is ▁being ▁used ▁as ▁a ▁gen etic ▁model ▁for ▁several ▁human ▁dise ases ▁including ▁the ▁ne uro de gener ative ▁dis orders ▁Park inson ' s , ▁Hun ting ton ' s , ▁spin oc ere bell ar ▁at ax ia ▁and ▁Al z heimer ' s ▁disease . ▁The ▁fly ▁is ▁also ▁being ▁used ▁to ▁study ▁mechan isms ▁underlying