Chromosome 21

Many genetic conditions are related to changes in particular genes on chromosome 21. This list of disorders associated with genes on chromosome 21 provides links to additional information.

Changes in the structure or number of copies of a chromosome can also cause problems with health and development. The following chromosomal conditions are associated with such changes in chromosome 21.

cancers

Rearrangements (translocations) of genetic material between chromosome 21 and other chromosomes have been associated with several types of cancer. For example, acute lymphoblastic leukemia (a type of blood cancer most often diagnosed in childhood) has been associated with a translocation between chromosomes 12 and 21. Another form of leukemia, acute myeloid leukemia, has been associated with a translocation between chromosomes 8 and 21.

Down syndrome

Down syndrome is a chromosomal condition that is associated with intellectual disability, a characteristic facial appearance, and weak muscle tone (hypotonia) in infancy. This condition is most often caused by trisomy 21. Trisomy 21 means that each cell in the body has three copies of chromosome 21 instead of the usual two copies.

Less commonly, Down syndrome occurs when part of chromosome 21 becomes attached (translocated) to another chromosome during the formation of reproductive cells (eggs and sperm) or very early in fetal development. Affected people have two copies of chromosome 21 plus extra material from chromosome 21 attached to another chromosome, resulting in three copies of genetic material from chromosome 21. Affected individuals with this genetic change are said to have translocation Down syndrome.

In a very small percentage of cases, Down syndrome results from an extra copy of chromosome 21 in only some of the body's cells. In these people, the condition is called mosaic Down syndrome.

Researchers believe that having extra copies of genes on chromosome 21 disrupts the course of normal development, causing the characteristic features of Down syndrome and the increased risk of health problems associated with this condition.

other chromosomal conditions

Other changes in the number or structure of chromosome 21 have a variety of effects on health and development. Chromosome 21 abnormalities can cause intellectual disability, delayed development, and characteristic facial features. In some cases, the signs and symptoms are similar to those of Down syndrome.

Changes involving chromosome 21 can include a missing segment of the chromosome in each cell (partial monosomy 21) and a circular structure called ring chromosome 21. A ring chromosome occurs when a chromosome breaks in two places and the ends of the chromosome arms fuse together to form a circular structure.

Geneticists use diagrams called ideograms as a standard representation for chromosomes. Ideograms show a chromosome's relative size and its banding pattern. A banding pattern is the characteristic pattern of dark and light bands that appears when a chromosome is stained with a chemical solution and then viewed under a microscope. These bands are used to describe the location of genes on each chromosome.

See How do geneticists indicate the location of a gene? in the Handbook.

You may find the following resources about chromosome 21 helpful. These materials are written for the general public.

• Additional NIH Resources - National Institutes of Health (2 links)
• MedlinePlus - Health information
  Encyclopedia: Chromosome

You may also be interested in these resources, which are designed for genetics professionals and researchers.

• Gene Tests - Genetic tests ordered by healthcare professionals
• Research Resources - Tools for researchers (6 links)
• PubMed - Recent literature
• Map Viewer - Genetic maps

The Handbook provides basic information about genetics in clear language.

• What is DNA?
• What is a chromosome?
• How many chromosomes do people have?
• Can changes in chromosomes affect health and development?

These links provide additional genetics resources that may be useful.

• Genetics education
• Human Genome Project
• Resources for Genetic Researchers

acute ; acute lymphoblastic leukemia ; acute myeloid leukemia ; base pair ; cancer ; cell ; chromosome ; DNA ; egg ; gene ; genome ; genome projects ; human genome ; human genome project ; hypotonia ; leukemia ; monosomy ; mosaic ; muscle tone ; myeloid ; rearrangement ; reproductive cells ; ring chromosomes ; sign ; sperm ; symptom ; syndrome ; translocation ; trisomy

You may find definitions for these and many other terms in the Genetics Home Reference Glossary.

See also Understanding Medical Terminology.