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The additional partial or full copy of the 21st chromosome which causes Down syndrome can originate from either the father or the mother. There is no definitive scientific research that indicates that Down syndrome is caused by environmental factors or the parents’ activities before or during pregnancy. However, due to higher birth rates in younger women, 80% of children with Down syndrome are born to women under 35 years of age. Maternal age is the only factor that has been linked to an increased chance of having a baby with Down syndrome resulting from nondisjunction or mosaicism. The cause of the extra full or partial chromosome is still unknown. This additional genetic material alters the course of development and causes the characteristics associated with Down syndrome. Regardless of the type of Down syndrome a person may have, all people with Down syndrome have an extra, critical portion of chromosome 21 present in all or some of their cells. This accomplishment opened the door to great advances in Down syndrome research. In the year 2000, an international team of scientists successfully identified and catalogued each of the approximately 329 genes on chromosome 21. It was later determined that an extra partial or whole copy of chromosome 21 results in the characteristics associated with Down syndrome. Instead of the usual 46 chromosomes present in each cell, Lejeune observed 47 in the cells of individuals with Down syndrome. In 1959, the French physician Jérôme Lejeune identified Down syndrome as a chromosomal condition. In recent history, advances in medicine and science have enabled researchers to investigate the characteristics of people with Down syndrome. Although other people had previously recognized the characteristics of the syndrome, it was Down who described the condition as a distinct and separate entity. It was this scholarly work, published in 1866, that earned Down the recognition as the “father” of the syndrome. It wasn’t until the late nineteenth century, however, that John Langdon Down, an English physician, published an accurate description of a person with Down syndrome. For centuries, people with Down syndrome have been alluded to in art, literature and science.