Parental origin of extra chromosomes in persons with X chromosome tetrasomy.

48,XXXX and 49,XXXXY chromosome constitutions are rare and while several such polysomies have been described in the past, the parental origin of the supernumerary chromosomes has only been described in a few cases.'2 More recently, difficulties owing to the reduced informativeness of the Xg antigen marker have been overcome by the use of X linked restriction fragment length polymorphisms (RFLPs) and, particularly, of multiallelic RFLPs. Using X linked DNA markers, we have analysed a group of four families with X chromosome tetrasomy, one with 48,XXXX and three with 49,XXXXY. The four probands had developmental retardation with at least some of the common dysmorphic features described in X chromo-