LA GL 1 ▁and ▁H Y MA I ▁are ▁blocked ▁or ▁not ▁expressed ▁by ▁D MR ▁m eth yl ation ▁and ▁only ▁the ▁father ' s ▁alle les ▁of ▁P LA G 1 ▁and ▁H Y MA 1 ▁are ▁expressed . ▁The ▁previously ▁listed ▁gen etic ▁mechan isms ▁result ▁in ▁twice ▁the ▁normal ▁amount ▁of ▁these ▁two ▁gen es ▁and ▁cause ▁chrom os ome ▁ 6 q 2 4 ▁T ND M . ▁ ▁Z FP 5 7 ▁Gene ▁ ▁Third , ▁mother ' s ▁hyp om eth yl ation ▁defect s ▁( a ▁gen etic ▁defect ▁that ▁stops ▁the ▁alle le ▁from ▁getting ▁ ▁a ▁m eth yl ▁group , ▁which ▁would ▁in hib it ▁trans cription ) ▁can ▁occur ▁from ▁an ▁isolated ▁genom ic ▁im print ing ▁or ▁occur ▁as ▁a ▁defect ▁called , ▁" h yp om eth yl ation ▁im print ed ▁lo ci " ▁( H IL ). ▁H IL ▁is ▁defined ▁as ▁the ▁loss ▁of ▁a ▁m eth yl ▁group ▁in ▁the ▁ 5 - m eth yl cy t os ine ▁nucle ot ide ▁at ▁a ▁fixed ▁position ▁on ▁a ▁chrom os ome . ▁H omo zy g ous ▁( ▁having ▁two ▁of ▁the ▁same ▁alle les ) ▁or ▁heter o zy g ous ▁( defined ▁as ▁having ▁one ▁each ▁of ▁two ▁different ▁alle les ) ▁Z FP 5 7 ▁path ogen ic ▁variant ▁make ▁up ▁almost ▁half ▁of ▁T ND M - H IL , ▁but ▁the ▁other ▁causes ▁of ▁H IL ▁are ▁unknown .