What known chromosomal reason could explain for autosomal recessive disorders where one parent is unaffected and the other is a carrier to cause the condition for the child?

An interesting corollary of uniparental disomy is that it can result in a child being affected by an autosomal recessive disorder when only one parent is a carrier for that disorder. The son or the daughter inherits two identical copies of the chromosome with the mutation (isodisomy) and so develops the disease. Uniparental disomy (UPD) occurs when a person receives two copies of a chromosome, or pa of a chromosome, from one parent and no copies from the other parent. Ref: Mosby's Medical Dictionary, 8th edition. (c) 2009, Elsevier; Nelson's Textbook of Paediatrics 17th Edition, Page 389; Metabolic Encephalopathies: Therapy and Prognosis By Stefano Di Donato, 1995, Page 198.