Down syndrome is caused by extra genetic material from chromosome 21. Chromosomes are the structures in cells that contain the genes.
Each person normally has 23 pairs of chromosomes, or 46 in all. An individual inherits one chromosome per pair from the mother’s egg and one from the father’s sperm. When an egg and sperm join together, they normally form a fertilized egg with 46 chromosomes.
Sometimes something goes wrong before fertilization. A developing egg or sperm cell may divide incorrectly, sometimes causing an egg or sperm cell to have an extra chromosome number 21. When this cell joins with a normal egg or sperm cell, the resulting embryo has 47 chromosomes instead of 46. Down syndrome is called trisomy 21 because affected individuals have three number 21 chromosomes, instead of two. This type of error in cell division causes about 95 percent of the cases of Down syndrome.
Occasionally, before fertilization, a part of chromosome 21 breaks off during cell division and becomes attached to another chromosome in the egg or sperm cell. The resulting embryo may have what is called translocation Down syndrome. Affected individuals have two normal copies of chromosome 21, plus extra chromosome 21 material attached to another chromosome. This type of error in cell division causes about 3 to 4 percent of the cases of Down syndrome. In some cases, the parent has a rearrangement of chromosome 21, called a balanced translocation, which does not affect his or her health.
About 1 to 2 percent of individuals with Down syndrome have a form called mosaicism. In this form, the error in cell division occurs after fertilization. Affected individuals have some cells with an extra chromosome 21 and others with the normal number.
The risk of Down syndrome increases with the mother’s age. Even though the risk is greater as the mother’s age increases, about 80 percent of babies with Down syndrome are born to women under age 35. This is because younger women have more babies than older women.