Please answer the following question: I read this background article the other day: One of the most common chromosome abnormalities is Down syndrome , due to nondisjunction of chromosome 21 resulting in an extra complete chromosome 21, or part of chromosome 21 ( Figure below ). Down syndrome is the only autosomal trisomy where an affected individual may survive to adulthood. Individuals with Down syndrome often have some degree of mental retardation, some impairment of physical growth, and a specific facial appearance. With proper assistance, individuals with Down syndrome can become successful, contributing members of society. The incidence of Down syndrome increases with maternal age. The risk of having a child with Down syndrome is significantly higher among women age 35 and older. I am facing a new situation today: Mary had two children. Her firs born was named Bill and was normal, and her second child was named Sam and had Down syndrome. Using the knowledge I acquired from the background article, how should I answer correctly the following question regarding my new situation: Which child didn't have a nondisjunction of chromosome 21?
A:
Bill