Think of it this way: certain genes are "stamped" or "imprinted" to be active only when inherited from a specific parent. UPD disrupts this delicate balance. This disruption can lead to severe imprinting disorders, such as the well-known and Angelman syndrome . These syndromes arise when both copies of a specific chromosome region are inherited from only one parent, leading to the loss or duplication of critical gene activity. UPD typically occurs as a random, unpreventable error during the formation of egg or sperm cells or in the earliest stages of fetal development.