▁multiple ▁re actions . ▁Development al ▁ple iot ropy ▁occurs ▁when ▁mut ations ▁have ▁multiple ▁effects ▁on ▁the ▁resulting ▁phen ot ype . ▁Selection al ▁ple iot ropy ▁occurs ▁when ▁the ▁resulting ▁phen ot ype ▁has ▁many ▁effects ▁on ▁fit ness ▁( dep ending ▁on ▁factors ▁such ▁as ▁age ▁and ▁gender ). ▁ ▁An ▁example ▁of ▁ple iot ropy ▁is ▁phen yl ket on uria , ▁an ▁inherited ▁dis order ▁that ▁affect s ▁the ▁level ▁of ▁phen yl al an ine ▁in ▁the ▁human ▁body . ▁Ph en yl al an ine ▁is ▁an ▁am ino ▁acid ▁that ▁can ▁be ▁obtained ▁from ▁food . ▁Ph en yl ket on uria ▁causes ▁this ▁am ino ▁acid ▁to ▁increase ▁in ▁amount ▁in ▁the ▁body , ▁which ▁can ▁be ▁very ▁dangerous . ▁The ▁disease ▁is ▁caused ▁by ▁a ▁defect ▁in ▁a ▁single ▁gene ▁on ▁chrom os ome ▁ 1 2 ▁that ▁codes ▁for ▁en zym e ▁phen yl al an ine ▁hydro x yl ase , ▁that ▁affect s ▁multiple ▁systems , ▁such ▁as ▁the ▁nerv ous ▁and ▁inte g ument ary ▁system . ▁Other ▁examples ▁of ▁ple iot ropy ▁are ▁al bin ism , ▁sick le ▁cell ▁an emia , ▁and ▁certain ▁forms ▁of ▁aut ism ▁and ▁sch iz op hren ia . ▁P lei ot ropy ▁not ▁only ▁affect s ▁humans , ▁but ▁also ▁animals , ▁such ▁as ▁ch ick ens ▁and ▁labor atory ▁house ▁m ice , ▁where ▁the ▁m ice ▁have ▁the ▁" min i - mus cle " ▁alle le .