Trisomy 21, or Down syndrome, occurs when a child has three copies of chromosome 21 instead of the typical two. It is not hereditary or caused by environmental factors, and there is no known cure. Diagnostic tests can be performed during pregnancy, and early intervention programs can help with development. Chromosome 21 is also related to other diseases and some cancers.
Trisomy 21 occurs when a child has three copies of chromosome 21 instead of the typical two in all of their cells. This type of abnormal cell division occurs during egg or sperm development. Trisomy 21 is by far the most common cause of Down syndrome, occurring in over 90% of cases. The condition is not hereditary, nor are there any known environmental or behavioral causes. As of 2011, there isn’t even a 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 instead of 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, oocytes and sperm cells each start with 46 chromosomes or 23 pairs before undergoing cell division. During this division, the 46 chromosomes are split in half so that the egg and sperm end up with 23 chromosomes, or one copy of each pair. When a sperm fertilizes an egg, the 23 chromosomes in each combine to form a complete set of 46 chromosomes, or 23 new pairs.
Trisomy 21 can occur when an egg or sperm with 46 chromosomes that is split in half retains both instead of one copy of chromosome 21. If this egg or sperm is later fertilized, the baby will have three copies of chromosome 21 in each cell: two from the abnormal cell and one from the normal cell. This extra copy of chromosome 21 causes Down syndrome.
Because trisomy 21 is caused by abnormal cell division, it is not considered a heritable condition. The source of this chromosomal error is not understood, but no scientific study suggests that either parent can do anything to cause or prevent it. There is no known cure for the genetic condition, but early intervention programs specifically for children with Down syndrome can help with the development of cognitive, sensory and motor skills.
A chromosomal abnormality such as trisomy 21 can be detected while a baby is in utero by analyzing the amniotic fluid or placenta cells. 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 tests are usually much more accurate than a fetal ultrasound but may present a risk of miscarriage.
Chromosome 21 is the smallest human chromosome and is believed to contain 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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