Down Syndrome is usually caused by an error in cell division called non disjunction. However, two other types of chromosomal abnormalities, mosaicism and translocation, are also implicated in Down Syndrome. Regardless of the type of Down Syndrome which a person may have, all people with Down Syndrome have an extra critical portion of the number 21 chromosome present in all, or some of their cells. This additional genetic material alters the course of development and causes the characteristics associated with the syndrome.

Non disjunction is a faulty cell division which results in an embryo with three number 21 chromosomes instead of two. Prior to, or at conception, a pair of number 21 chromosomes in either the sperm or the egg fail to separate. As the embryo develops, the extra chromosome is replicated in every cell of the body. Because there are three copies of the 21st chromosome, this form is refered to as "Trisomy 21." This faulty cell division is responsible for 95 percent of all cases of Down Syndrome.

Mosaicism occurs when non disjunction of the 21 st chromosome takes place in one of the initial cell divisions after fertilization. When this occurs, there is a mixture of two types of cells, some containing 46 chromosomes and some containing 47. Those cells with 47 chromosomes contain an extra 21st chromosome. Because of the "mosaic" pattern of the cells, the term mosaicism is used. Mosaicism is rare, being responsible for only one to two percent of all cases of Down Syndrome.

Translocation is a different type of chromosomal problem and occurs in only three to four percent of people with Down Syndrome. Translocation occurs when part of the number 21 chromosome breaks off during cell division and attaches to another chromosome. While the total number of chromosomes in the cells remains 46, the presence of an extra part of the number 21 chromosome causes the features of Down Syndrome. As with non disjunction trisomy 21, translocation occurs either prior to or at conception.