<prompt> 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?Germline mosaicismGenomic imprintingPenetrationUniparental disomy  <response>: If one parent is normal for an autosomal recessive disorder and the other is carrier - No child should be affected. But there is exception to this rule, i.e. uniparental disomy UPD, when a person receives 2 copies of a chromosome, from one parent and no copies from the other parent. Ex: In Prader-Willi syndrome- maternal UPD of chromosome 15 missing the paternal chromosome 15. In Angelman syndrome- paternal UPD of chromosome 15 missing the maternal chromosome 15. 