Human Heredity 9th Edition by Michael Cummings

Exercise 3

A genetic counselor was called to the nursery for consultation on a newborn described as "floppy with a weak cry." The counselor noted that the newborn's chart indicated that he was having feeding problems and had not gained weight since his delivery 15 days earlier. The counselor noted several other findings during her evaluation. The infant had almond-shaped eyes, a small mouth with a thin upper lip, downturned corners of the mouth, and a narrow face. He was born with undescended testes and a small penis. The counselor suspected that this child had Prader-Willi syndrome.
Prader-Willi syndrome is caused by a deletion or a mutation on the long arm of chromosome 15 or by UPD. Prader-Willi can hap-pen three ways: a deletion on the father's copy of chromosome 15, a mutation in the Prader-Willi gene on the father's copy of chromo-some 15, or maternal UPD, where both copies of chromosome 15 are from the mother and none are contributed by the father.
The child and his parents were tested for a deletion in the long arm of chromosome 15 (15q11-q13) by fluorescence in situ hybridization (FISH) and for uniparental disomy 15 by polymerase chain reaction (PCR). In this case, maternal disomy-which is the cause of Prader-Willi syndrome in about 30% of the cases was detected by PCR.
Explain to the parents how maternal disomy happens during gamete formation and/or in mitosis after fertilization.