Humans usually have 46 chromosomes (23 pairs) in each cell.
Two copies of chromosome 22, one copy inherited from each parent, form one of the pairs.
Chromosome 22 is the second smallest human chromosome, encompassing more than 51 million DNA building blocks (base pairs), and represents between 1.5 and 2 percent of the total DNA in cells.
In 1999, researchers working on the Human Genome Project announced that they had determined the sequence of base pairs that make up this chromosome. Chromosome 22 was the first human chromosome to be sequenced entirely.
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 22 likely contains 500 to 600 genes that provide instructions for making proteins. These proteins perform a variety of different roles in the body.
Related health conditions
The following chromosomal conditions are associated with changes in the structure or number of copies of chromosome 22.
22q11.2 deletion syndrome
22q11.2 deletion syndrome is a disorder that involves heart defects, an opening in the roof of the mouth (cleft palate), distinctive facial features, low calcium levels, and an increased risk of behavior problems and mental illnesses such as schizophrenia.
An extra copy of genetic material causes the 22q11.2 duplication at position q11.2 on chromosome 22. In most cases, this additional genetic material consists of a sequence of approximately 3 million base pairs, also written as three megabases (Mb).
22q13.3 deletion syndrome
The 22q13.3 deletion syndrome, also commonly known as Phelan-McDermid syndrome, is caused by a deletion near the end of the long arm (q) of chromosome 22. A chromosome 22 ring can also cause 22q13 deletion syndrome. .3.
A ring chromosome is a circular structure that occurs when the chromosome breaks in two places, the ends of the chromosome are lost, and the broken ends fuse. People with ring chromosome 22 have a copy of this abnormal chromosome in some or all of their cells.
The researchers believe that several critical genes near the end of the q arm of chromosome 22 are lost when the ring chromosome 22 is formed. If the breaking point on the long arm is at chromosome 22q13.3, people with ring chromosome 22 will experience signs and symptoms similar to those with a simple deletion.
The signs and symptoms of 22q13.3 deletion syndrome are likely related to the loss of multiple genes at the end of chromosome 22.
The loss of a particular gene, SHANK3, is believed to be responsible for many characteristics of 22q13.3 deletion syndrome, such as developmental delay, intellectual disability, and absent or severely delayed speech. Additional genes in the deleted region likely contribute to the signs and symptoms of 22q13.3 deletion syndrome.
Chronic myeloid leukemia
A rearrangement (translocation) of genetic material between chromosomes 9 and 22 causes a type of blood-forming cell cancer called chronic myeloid leukemia.
This slow-growing cancer leads to an overproduction of abnormal white blood cells. Standard features of the condition include excessive tiredness (fatigue), fever, weight loss, and an enlarged spleen.
Opitz G / BBB syndrome
A deletion in one copy of chromosome 22 can cause Opitz G / BBB syndrome. This condition causes several abnormalities along the body’s midline, including widely spaced eyes (ocular hypertelorism), difficulty breathing or swallowing, brain malformations, distinctive facial features, and genital abnormalities in men.
The deletion that causes Opitz G / BBB syndrome is in the same area as the deletion that causes the 22q11.2 deletion syndrome (described above), so Opitz G / BBB syndrome is often considered part of the syndrome 22q11.2 deletion. It is not yet known which deleted genes cause the signs and symptoms of Opitz G / BBB syndrome.
A small percentage of individuals with schizophrenia have a small deletion ( microdeletion ) in a region of chromosome 22 called 22q11. Schizophrenia is a mental health disorder that affects a person’s thinking, sense of personality, and perceptions.
The 22q11 region contains several genes that are believed to affect the risk of schizophrenia. Losing one or more of these genes can affect the brain to increase the risk of developing this disorder. However, the relationships between these gene losses and the development of the disease are not well understood.
In addition to schizophrenia, some people with this deletion have additional signs and symptoms that include 22q11.2 deletion syndrome (described above).
22q11 microdeletions are among the many factors being studied to help explain the causes of schizophrenia. A host of genetic and environmental factors, most of which are still unknown, likely contribute to the risk of developing this complex condition.
Other chromosomal conditions
Other changes in the amount or structure of chromosome 22 can have various effects. Standard features are intellectual disability, developmental delay, delayed or absent speech, distinctive facial features, and behavior problems.
Expected changes to chromosome 22 include an extra piece of chromosome in each cell ( partial trisomy ), a missing segment of the chromosome in each cell (partial monosomy), and a chromosome 22 in the ring.
Translocations of genetic material between chromosomes can also lead to extra or missing material from chromosome 22. The most common of these translocations involves chromosomes 11 and 22.
Cat-eye syndrome is a rare disorder most often caused by a chromosomal change called inverted duplication.
In people with this condition, each cell has at least one extra small chromosome made up of genetic material from chromosome 22 that has been abnormally duplicated.