Reviewed January 2009
What is chromosome 18?
Humans normally have 46 chromosomes in each cell, divided into 23 pairs. Two copies of chromosome 18, one copy inherited from each parent, form one of the pairs. Chromosome 18 spans about 76 million base pairs (the building blocks of DNA) and represents approximately 2.7 percent of the total DNA in cells.
Identifying genes on each chromosome is an active area of genetic research. Because researchers use different approaches to predict the number of genes on each chromosome, the estimated number of genes varies. Chromosome 18 likely contains between 300 and 500 genes.
Genes on chromosome 18 are among the estimated 20,000 to 25,000 total genes in the human genome.
There are many genetic conditions related to genes on chromosome 18.
What chromosomal conditions are related to chromosome 18?
The following conditions are caused by changes in the structure or number of copies of chromosome 18.
-
trisomy 18
-
Trisomy 18 occurs when each cell in the body has three copies of chromosome 18 instead of the usual two copies. Trisomy 18 can also result from an extra copy of chromosome 18 in only some of the body's cells (mosaic trisomy 18).
Rarely, trisomy 18 occurs when the long (q) arm of chromosome 18 becomes attached (translocated) to another chromosome during the formation of reproductive cells (eggs and sperm) or very early in embryonic development. Affected individuals have two copies of chromosome 18, plus extra material from chromosome 18 attached to another chromosome. People with this genetic change are said to have translocation trisomy 18. If only part of the q arm is present in three copies, the physical signs of translocation trisomy 18 may be different from those typically seen in trisomy 18. If the entire q arm is present in three copies, individuals may be as severely affected as if they had three full copies of chromosome 18.
Researchers believe that extra copies of some genes on chromosome 18 disrupt the course of normal development, causing the characteristic features of trisomy 18 and the increased risk of medical problems associated with this disorder.
- other chromosomal conditions
-
Other disorders associated with chromosome 18 occur when pieces of either the long (q) arm or short (p) arm of this chromosome are missing or when extra genetic material from chromosome 18 is present. Researchers are uncertain how missing or extra pieces of chromosome 18 lead to the features of these disorders.
Chromosome 18p partial monosomy occurs when a piece of the short arm of this chromosome is deleted. Individuals with this condition often have short stature, a round face, large ears, a shortened space between the nose and mouth (philtrum), droopy eyelids (ptosis), and mild to moderate intellectual disability. About 10 to 15 percent of people with this condition have serious abnormalities of the brain and spinal cord (central nervous system). The lifespan of individuals with chromosome 18p partial monosomy is typically not reduced, except when severe brain abnormalities are present.
Chromosome 18q partial monosomy occurs when a piece of the long arm of this chromosome is deleted. The signs and symptoms of this condition vary widely among affected individuals. People with chromosome 18q partial monosomy may have short stature, skeletal abnormalities, mild to severe intellectual disability, hearing loss, and problems coordinating movements. The lifespan of individuals with this condition depends on the disease severity.
Extra pieces of the short arm of chromosome 18 can result in a disorder called tetrasomy 18p. This condition occurs when a person has an additional chromosome called an isochromosome. This isochromosome is made up of two copies of the short arm of chromosome 18 and no copies of the long arm. As a result, cells have four copies of the short arm of chromosome 18 instead of the usual two copies. Individuals with this condition are usually small for their age and may have low-set ears, a small head (microcephaly), a high arch in the roof of the mouth (high-arched palate), a small mouth, and mild to severe intellectual disability. The lifespan of people with tetrasomy 18p depends on the disease severity.
Some people have a chromosome 18 with a circular structure, which is called a ring chromosome 18. This type of chromosome is formed when breaks occur at both ends of the chromosome and the broken ends join together to form a ring. Individuals with this chromosome abnormality often have intellectual disability, microcephaly, widely spaced eyes (hypertelorism), low-set ears, and speech problems. The severity of symptoms in people with ring chromosome 18 depends on how much genetic material is lost from each arm of the chromosome.
Is there a standard way to diagram chromosome 18?
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.
Where can I find additional information about chromosome 18?
You may find the following resources about chromosome 18 helpful. These materials are written for the general public.
- Additional NIH Resources - National Institutes of Health
-
- MedlinePlus - Health information
You may also be interested in these resources, which are designed for genetics professionals and researchers.
Where can I find general information about chromosomes?
The Handbook provides basic information about genetics in clear language.
These links provide additional genetics resources that may be useful.
What glossary definitions help with understanding chromosome 18?
The resources on this site should not be used as a substitute for
professional medical care or advice. Users seeking information about
a personal genetic disease, syndrome, or condition should consult with a qualified
healthcare professional.
See How can I find a genetics professional in my area? in the Handbook.
Indicates a page outside Genetics Home Reference.
