Uniparental disomy has been recognized for most chromosomes. For some, like chromosomes 3, 12, 13, 18, 19 and 21, there is no apparent phenotypic consequence (Fig. 2). For others, like chromosomes 2, 4, 5, 8, 9, and 10, there may be, and sporadic cases have been described [5]. For chromosomes 6, 7, 11, 14, 15, 16 and 20, it has been well documented that UPD causes abnormality (Ta­ble 2) [5,11]. Kalousek and Barrett [12]caution that a phenotypic defect may be due to an accompanying placen­tal trisomy or undetected low-level mosaicism in the fetus rather than to the UPD itself. This might be a major reason for both normal and abnormal phenotypes in cases with UPD for the same chromosome.

Table 1. Formation of uniparental disomy.