What Causes Trisomy 21?

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  • Written By: C. K. Lanz
  • Edited By: Melissa Wiley
  • Last Modified Date: 13 September 2019
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Trisomy 21 results when a baby has three rather than the typical two copies of chromosome 21 in all of his or her cells. This type of abnormal cell division occurs during the development of the egg or sperm cell. Trisomy 21 is by far the most common cause of Down syndrome, as it occurs in more than 90 percent of cases. The condition is not hereditary, nor are there any known environmental or behavioral causes. As of 2011, there is also no cure.

A normal human cell contains 23 pairs of chromosomes, or threadlike strands of deoxyribonucleic acid (DNA) that carry genes in a linear order. Each pair consists of one chromosome from the person’s mother and the other from the father, for a total of 46 chromosomes. A trisomy occurs when there are three copies of a chromosome rather than the normal two. Therefore, a person born with three rather than two copies of chromosome 21 in each cell has trisomy 21.

During a typical reproduction, the egg and sperm cells each begin with 46 chromosomes or 23 pairs before undergoing cell division. During this division, the 46 chromosomes are divided in half so that the egg and sperm cell each finish with 23 chromosomes, or one copy of every pair. When a sperm cell fertilizes an egg cell, the 23 chromosomes in each combine to make a complete set of 46 chromosomes, or 23 new pairs.


Trisomy 21 can occur when an egg or sperm cell with 46 chromosomes that is being divided in half keeps both instead of one copy of chromosome 21. If this egg or sperm cell is then fertilized, the child will have three copies of chromosome 21 in every cell: two from the abnormal cell and one from the normal cell. This extra copy of chromosome 21 causes Down syndrome.

As trisomy 21 is caused by abnormal cell division, it is not considered an inheritable condition. The source of this chromosomal error is not understood, but no scientific study suggests that either parent can do something to cause or prevent it. There is no known cure for the genetic condition, but early intervention programs specific to children with Down syndrome can help with the development of cognitive, sensory, and motor skills.

A chromosomal abnormality like trisomy 21 can be detected while a child is in utero by analyzing the amniotic fluid or cells from the placenta. Amniocentesis, chorionic villus sampling, and percutaneous umbilical blood sampling are all diagnostic tests that can be performed during pregnancy to diagnose Down syndrome. These types of analysis are generally much more accurate than a fetal ultrasound but can present a risk of miscarriage.

Chromosome 21 is the smallest human chromosome and is believed to contain approximately 300 to 400 genes. In addition to Down syndrome, genes on chromosome 21 are related to other diseases, including Alzheimer’s, Romano-Ward syndrome, and nonsyndromic deafness. Some cancers, such as acute lymphoblastic leukemia, have been associated with translocations of genetic material between other chromosomes and chromosome 21.


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