Summary. Uniparental disomy (UPD) is the presence, in a diploid individual, of a chromosome pair that derives from only one parent. In general the mechanism of formation of UPD is based on the correction of aneuplody derived from meiotic or mitotic errors. UPD almost all chromosmes have been documented but preponderance of maternal versus paternal disomy (3:1) have been observed. This difference can be explained with the frequency of aneuploidy in the female gametogenesis.