ia ▁and ▁b ip olar ▁dis order . ▁These ▁particular ▁studies ▁show ▁clust ering ▁of ▁these ▁dise ases ▁within ▁patients ▁themselves ▁or ▁families . ▁The ▁estimated ▁her it ability ▁of ▁sch iz op hren ia ▁is ▁ 7 0 % ▁to ▁ 9 0 %, ▁therefore ▁the ▁ple iot ropy ▁of ▁gen es ▁is ▁cru cial ▁since ▁it ▁causes ▁an ▁increased ▁risk ▁for ▁certain ▁psych otic ▁dis orders ▁and ▁can ▁aid ▁psych iat ric ▁diagn osis . ▁ ▁Ph en yl ket on uria ▁( PK U ) ▁▁ ▁A ▁common ▁example ▁of ▁ple iot ropy ▁is ▁the ▁human ▁disease ▁phen yl ket on uria ▁( PK U ). ▁This ▁disease ▁causes ▁mental ▁ret ard ation ▁and ▁reduced ▁hair ▁and ▁skin ▁p ig ment ation , ▁and ▁can ▁be ▁caused ▁by ▁any ▁of ▁a ▁large ▁number ▁of ▁mut ations ▁in ▁the ▁single ▁gene ▁on ▁chrom os ome ▁ 1 2 ▁that ▁codes ▁for ▁the ▁en zym e ▁phen yl al an ine ▁hydro x yl ase , ▁which ▁converts ▁the ▁am ino ▁acid ▁phen yl al an ine ▁to ▁ty ros ine . ▁Depending ▁on ▁the ▁mut ation ▁involved , ▁this ▁conversion ▁is ▁reduced ▁or ▁ce ases ▁entirely . ▁Un convert ed ▁phen yl al an ine ▁builds ▁up ▁in ▁the ▁blood stream ▁and ▁can ▁lead ▁to ▁levels ▁that ▁are ▁to xic ▁to ▁the ▁developing ▁nerv ous ▁system ▁of ▁new born ▁and ▁infant ▁children . ▁The ▁most ▁dangerous ▁form ▁of ▁this ▁is ▁called ▁classic ▁PK U , ▁which ▁is ▁common ▁in ▁inf ants