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?: (A) Germline mosaicism, (B) Genomic imprinting, (C) Penetration, (D) Uniparental disomy
Answer is D. 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).