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 78 million DNA building blocks (base pairs) and represents approximately 2.5 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 200 to 300 genes that provide instructions for making proteins. These proteins perform a variety of different roles in the body.
The following chromosomal conditions are associated with changes in the structure or number of copies of chromosome 18.
18q deletion syndrome is caused by a deletion of genetic material from the long (q) arm of chromosome 18. This chromosomal change is written as 18q-. The signs and symptoms of 18q deletion syndrome are probably related to the loss of multiple genes in this region. Researchers are working to determine how the loss of these genes leads to the signs and symptoms of the disorder, which is characterized by neurological abnormalities including intellectual disability or learning problems and a wide variety of other features. Some affected individuals have a loss of tissue called white matter in the brain and spinal cord (leukodystrophy).
18q deletion syndrome is often categorized into two types: individuals with deletions near the end of the long arm of chromosome 18 are said to have distal 18q deletion syndrome, and those with deletions in the part of the long arm near the center of chromosome 18 are said to have proximal 18q deletion syndrome. The signs and symptoms of these two types of the condition are overlapping, with certain features being more common in one form of the disorder than in the other. For example, hearing loss and heart abnormalities are more common in people with distal 18q deletion syndrome, while seizures occur more often in people with proximal 18q deletion syndrome.
Tetrasomy 18p results from the presence of an abnormal extra chromosome, called an isochromosome 18p, in each cell. An isochromosome is a chromosome with two identical arms. Normal chromosomes have one long (q) arm and one short (p) arm, but isochromosomes have either two q arms or two p arms. Isochromosome 18p is a version of chromosome 18 made up of two p arms.
Cells normally have two copies of each chromosome, one inherited from each parent. In people with tetrasomy 18p, cells have the usual two copies of chromosome 18 plus an isochromosome 18p. As a result, each cell has four copies of the short arm of chromosome 18. (The word "tetrasomy" is derived from "tetra," the Greek word for "four.") The extra genetic material from the isochromosome disrupts the normal course of development, causing intellectual disability, delayed development, and the other characteristic features of this disorder.
Trisomy 18 occurs when each cell in the body has three copies of chromosome 18 instead of the usual two copies, causing severe intellectual disability and multiple birth defects that are usually fatal by early childhood. (The word "trisomy" comes from "tri," the Greek word for "three.") In some cases, the extra copy of chromosome 18 is present in only some of the body's cells. This condition is known as mosaic trisomy 18.
Rarely, trisomy 18 is caused by an extra copy of only a piece of chromosome 18. This condition is known as partial trisomy 18. Partial trisomy 18 occurs when part of 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 the extra material from chromosome 18 attached to another chromosome. If only part of the q arm is present in three copies, the physical signs of partial trisomy 18 may be less severe than 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 health problems associated with this disorder.
Other disorders associated with chromosome 18 occur when pieces of the 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 specific features of these disorders.
Partial monosomy of chromosome 18p (18p-) 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 partial monosomy of chromosome 18p is typically not reduced, except when severe brain abnormalities are present.
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, an unusually small head (microcephaly), widely spaced eyes (hypertelorism), low-set ears, and speech problems. The signs and symptoms associated with ring chromosome 18 depend on how much genetic material is lost from each arm of the chromosome.
Geneticists use diagrams called idiograms as a standard representation for chromosomes. Idiograms show a chromosome's relative size and its banding pattern, which 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.