Down Syndrome is named after John Langdon Down, a British doctor who first studied and described the mental disorder. He discovered the disorder in Surrey, England, while working at an asylum for mentally retarded children. He called people affected by this disorder Mongoloids due to the physical similarities of Mongolian citizens compared to those affected by Down syndrome. Subsequently, the term “Mongoloid” was abandoned and named after John Down when Jerome Lejeune, a French geneticist who tested children with these Mongolian-like physical characteristics, found that 97% of those tested had a chromosome-21 in more, for a total of 47 chromosomes. Down syndrome is a common disorder due to an extra number of chromosomes; it is the most common cause of mental retardation occurring in 1 in 1,000 children in the United States. (Genetic Sciences Learning Center) There are three types of Down syndrome: trisomy 21, translocation, and mosaicism. Trisomy 21 is the most common type of Down syndrome; occurs in 95% of Down syndrome cases. Trisomy 21 is a disorder in which a newborn receives an extra third chromosome 21, instead of the normal 2. The second type of Down syndrome is translocation; occurs when only a small piece of chromosome 21 is found on another chromosome. Only 3% of people with Down syndrome suffer from translocation. Mosaicism is the third type of Down syndrome and also the rarest; is a disorder in which newborns have a mix of cells on chromosome-47 that contains an extra chromosome-21. Only 2% of people with Down syndrome have mosaicism. (Hauser-Cram, 15) The causes of Down syndrome are still unknown. There is no way to prevent the onset of Down syndrome,...... half of the article......://learn.genetics.utah.edu/content/disorders/whataregd/down/ >Hauser-Cram, Penny. Children with disabilities: A longitudinal study of child development and parental well-being. Boston: Blackwell, 2001. Kumin, Libby. Language skills in the classroom for children with Down syndrome: A guide for parents and teachers. 1st ed. Maryland: Woodbine House, 2001. Leshin, Dr. Len. Down syndrome: health problems. May 2009.McGuire, Dennis. Mental well-being in adults with Down syndrome: A guide to emotional and behavioral strengths and challenges. 1st ed. Maryland: WoodbineHouse, 2006. Skallerup, Susan, ed. Children with Down syndrome. 3rd ed. Boston: Woodbine House, 2008. Skotko, Brian. Lace Up Your Pace: A Crash Course in Down Syndrome for Brothers and Sisters. 1st ed. Maryland: Woodbine House, 2009.