In an Autosomal Recessive (AR) disorder, one parent is normal and the other is carrier and the child is also affected. What is the reason

Uniparental disomy refers to the situation in which 2 copies of a chromosome come from the same parent, instead of 1 copy coming from the mother, and 1 copy coming from the father. Angelman syndrome (AS) and Prader-Willi syndrome (PWS) are examples of disorders that can be caused by uniparental disomy. Reference: GHAI Essential pediatrics, 8th edition