A typical human cell contains 23 pairs of chromosomes, with half inherited from each parent. Down Syndrome is a genetic condition that occurs when a child has a full or partial extra copy of chromosome 21. The transformed genetic material alters the course of development and causes the characteristics associated with Down Syndrome. According to the Centers for Disease Control and Prevention, approximately one in every 700 babies in the United States is born with Down Syndrome.
Most cases of Down Syndrome are spontaneous and sporadic events. Maternal age is the only factor linked to an increased likelihood of having a baby with Down syndrome; however, 80% of children with Down Syndrome are born to women under the age of 35. Down Syndrome is usually identified at birth by certain physical traits including low muscle tone, a single deep crease across the palm of the hand, a slightly flattened facial profile and an upward slant to the eyes. These features may be present in babies without Down Syndrome, so a chromosomal analysis called a karyotype, consisting of a blood draw, is commonly done to confirm the diagnosis.
There are two known types of Down Syndrome: Trisomy 21, where an embryo develops with three copies of chromosome 21 instead of the usual two, and Mosaicism, which is diagnosed when there is a mixture of two types of cells, some containing the usual 46 chromosomes and some containing 47. Those cells with 47 chromosomes contain an extra chromosome 21. Mosaicism is much less common than Trisomy 21, accounting for only about one percent of all cases of Down Syndrome. Children with Down Syndrome possess some degree of cognitive delay, ranging from mild to severe.