Down Syndrome

Down syndrome is the most common cause of mental retardation. It can be caused by the presence of an extra chromosome. Chromosomes contain sequences of DNA called genes, which represent the genetic information that exists within almost every cell of the body. Twenty-three distinctive pairs, or 46 total chromosomes, are located within the nucleus (central DNA-containing structure) of each cell. When a sperm cell fertilizes an egg cell, the newly created zygote receives 23 chromosomes from each parent, for a total of 46 chromosomes. An aberration that occurs during division of the sperm or egg cell can cause this cell to have an extra chromosome for a total of 24 chromosomes instead of 23. This event occurs during cell division and is referred to as nondisjunction, or the failure of all chromosomes to separately properly resulting in retention of one of the chromosomes in one of the two new daughter cells and the loss of a chromosome in the other. Loss of a chromosome usually means that the embryo is nonviable. An embryo with an extra chromosome in all its cells (for a total of 47 chromosomes) is usually inconsistent with life. If the extra chromosome is number 21, and the fetus survives to term, the baby with have Down syndrome. This form of Down Syndrome is also called Trisomy 21 and An older sibling plays with her younger sister who has Down syndrome. Photograph by A. Sieveing. A. Sieveing/Petit Format/Photo Researchers, Inc. Reproduced by permission. accounts for approximately 95% of all Down syndrome patients.

In a very rare number of Down syndrome cases, the original egg and sperm cells begins with the correct number of chromosomes but shortly after fertilization during the phase where cells are dividing rapidly, a single cell can divide abnormally, creating a line of cells with an extra chromosome 21. This is called a cell line mosaicism. The individual with this type of Down syndrome has two types of cells: some with 46 chromosomes (the normal number), and some with 47 chromosomes (causing Down syndrome symptomatology). Individuals who are mosaic for Trisomy 21 typically have less severe signs and symptoms of the disorder.

Another relatively rare genetic abnormality that can cause Down syndrome is called a chromosome translocation. This is an event that unlike the numerical abnormality causing Trisomy 21, there is a structural abnormality. Exchange of material from two different chromosomes during the production of sex cells can take place such that there is a whole chromosome 21 attached to another chromosome but the chromosome number is normal. These types of translocations, involving chromosome 21, occur in about 3–4% of cases of Down syndrome.

Down syndrome occurs in about one in every 800 liveborns. It affects an equal number of male and female babies. The majority of cases of Down syndrome occur due to a nondisjunction event that occurs in the maternal sex cells. As the maternal age increases, the risk of having a Down syndrome baby increases significantly. For example, at younger ages, the risk is about one in 1,250. By the time the woman is 35 years old, the risk increases to one in 385; by age 40 the risk increases to one in 100; and by age 45 the risk is one in 25. There is no known maternal age-related increased risk if down syndrome results from either a cell line mosaicism or translocation.