Background: 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.

Situation: Norm and Ian were two brothers. Norm was a totally normal individual, while Ian had Down syndrome. Their parents loved them both equally.

Question: Which brother had a nondisjunction of chromosome 21?
Answer: Ian

Situation: Norm and Ian were two brothers. Norm was a totally normal individual, while Ian had Down syndrome. Their parents loved them both equally.

Question: Which brother didn't have a nondisjunction of chromosome 21?
Answer: Norm

Situation: Norm and Ian were two brothers. Norm was a totally normal individual, while Ian had Down syndrome. Their parents loved them both equally.

Question: Which brother didn't have an extra complete chromosome 21?
Answer: Norm

Situation: Norm and Ian were two brothers. Norm was a totally normal individual, while Ian had Down syndrome. Their parents loved them both equally.

Question: Which brother had an extra complete chromosome 21?
Answer: Ian

Situation: Norm and Ian were two brothers. Norm was a totally normal individual, while Ian had Down syndrome. Their parents loved them both equally.

Question: Which brother had an autosomal trisomy ?
Answer: Ian

Situation: Norm and Ian were two brothers. Norm was a totally normal individual, while Ian had Down syndrome. Their parents loved them both equally.

Question: Which brother didn't have an autosomal trisomy ?
Answer: Norm

Situation: Norm and Ian were two brothers. Norm was a totally normal individual, while Ian had Down syndrome. Their parents loved them both equally.

Question: Which brother didn't have a degree of mental retardation?
Answer: Norm

Situation: Norm and Ian were two brothers. Norm was a totally normal individual, while Ian had Down syndrome. Their parents loved them both equally.

Question: Which brother had a degree of mental retardation?
Answer: Ian

Situation: Norm and Ian were two brothers. Norm was a totally normal individual, while Ian had Down syndrome. Their parents loved them both equally.

Question: Which brother had some impairment of physical growth?
Answer: Ian

Situation: Norm and Ian were two brothers. Norm was a totally normal individual, while Ian had Down syndrome. Their parents loved them both equally.

Question: Which brother had no impairment of physical growth?
Answer: Norm

Situation: Mary had two children. Her firs born was named Bill and was normal, and her second child was named Sam and had Down syndrome.

Question: Which child had a nondisjunction of chromosome 21?
Answer: Sam

Situation: Mary had two children. Her firs born was named Bill and was normal, and her second child was named Sam and had Down syndrome.

Question: Which child didn't have a nondisjunction of chromosome 21?
Answer: Bill

Situation: Mary had two children. Her firs born was named Bill and was normal, and her second child was named Sam and had Down syndrome.

Question: Which child didn't have an autosomal trisomy ?
Answer: Bill

Situation: Mary had two children. Her firs born was named Bill and was normal, and her second child was named Sam and had Down syndrome.

Question: Which child had an autosomal trisomy ?
Answer: Sam

Situation: Mary had two children. Her firs born was named Bill and was normal, and her second child was named Sam and had Down syndrome.

Question: Which child had a degree of mental retardation?
Answer: Sam

Situation: Mary had two children. Her firs born was named Bill and was normal, and her second child was named Sam and had Down syndrome.

Question: Which child didn't have a degree of mental retardation?
Answer: Bill

Situation: Mary had two children. Her firs born was named Bill and was normal, and her second child was named Sam and had Down syndrome.

Question: Which child didn't have some impairment of physical growth?
Answer: Bill

Situation: Mary had two children. Her firs born was named Bill and was normal, and her second child was named Sam and had Down syndrome.

Question: Which child had some impairment of physical growth?
Answer: Sam

Situation: Mary had two children. Her firs born was named Bill and was normal, and her second child was named Sam and had Down syndrome.

Question: Which child had a specific facial appearance?
Answer: Sam

Situation: Mary had two children. Her firs born was named Bill and was normal, and her second child was named Sam and had Down syndrome.

Question: Which child didn't have a specific facial appearance?
Answer: Bill