Chromosome 21

Chromosome 21 is one of the 23 pairs of chromosomes in humans. Humans normally have two copies of this chromosome. Chromosome 21 spans about 48 million base pairs (the building material of DNA) and represents approximately 1.5 percent of the total DNA in cells.

Overview[edit | edit source]

Most researchers agree that Chromosome 21 is the smallest human autosome. It is estimated to contain 200 to 400 genes. However, because of the techniques available for identifying genes, the exact number of genes on each chromosome may not be known for some time.

Genes[edit | edit source]

Among the diseases related to genes on chromosome 21, the most significant is Down syndrome, also known as trisomy 21. It is caused by an extra copy of chromosome 21. Other medical conditions associated with genes on chromosome 21 include Alzheimer's disease, amyotrophic lateral sclerosis, and congenital heart disease.

Down Syndrome[edit | edit source]

Down syndrome is a condition in which a person has an extra chromosome. Chromosomes are small “packages” of genes in the body. They determine how a baby’s body forms during pregnancy and how the baby’s body functions as it grows in the womb and after birth. Typically, a baby is born with 46 chromosomes. But in Down syndrome, a baby is born with an extra copy of chromosome 21. This extra copy changes how the baby’s body and brain develop, which can cause both mental and physical challenges for the baby.

Research[edit | edit source]

Chromosome 21 is a focus of medical research for a variety of reasons. Scientists are making progress in identifying the genes on chromosome 21. In addition to those genes that cause Down syndrome, researchers have also located genes that may be responsible for other conditions. These include the development of cancer, heart disease, and mental illness.