Reviewed November 2006
What is chromosome 15?
Humans normally have 46 chromosomes in each cell, divided into 23 pairs. Two copies of chromosome 15, one copy inherited from each parent, form one of the pairs. Chromosome 15 spans about 100 million base pairs (the building blocks of DNA) and represents more than 3 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 15 likely contains between 650 and 1,000 genes.
Genes on chromosome 15 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 15.
What chromosomal conditions are related to chromosome 15?
The following conditions are caused by changes in the structure or number of copies of chromosome 15.
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Angelman syndrome
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Angelman syndrome results from a loss of gene activity in a specific part of chromosome 15 (the 15q11-q13 region) in each cell. This region contains a gene called UBE3A that, when mutated or absent, likely causes the characteristic features of this condition. People normally inherit one copy of the UBE3A gene from each parent. Both copies of this gene are active in many of the body's tissues. In the brain, however, only the copy inherited from a person's mother (the maternal copy) is active. This parent-specific gene activation is called genomic imprinting. If the maternal copy is lost because of a chromosomal change or a gene mutation, a person will have no working copies of the UBE3A gene in the brain.
In most cases (about 70 percent), people with Angelman syndrome have a deletion in the maternal copy of chromosome 15. This chromosomal change deletes the region of chromosome 15 that includes the UBE3A gene. Because the copy of the UBE3A gene inherited from a person's father (the paternal copy) is normally inactive in the brain, a deletion in the maternal chromosome 15 leaves no active copies of the UBE3A gene in the brain.
In 3 percent to 7 percent of cases of Angelman syndrome, people inherit two copies of chromosome 15 from their father instead of one copy from each parent. This phenomenon is called paternal uniparental disomy (UPD). People with paternal UPD for chromosome 15 have two copies of the UBE3A gene, but they are both inherited from the father and therefore are inactive in the brain.
About 10 percent of Angelman syndrome cases are caused by a mutation in the UBE3A gene, and another 3 percent result from a defect in the DNA region that controls the activation of the UBE3A gene and other genes on the maternal copy of chromosome 15. In a small percentage of cases, Angelman syndrome may be caused by a chromosomal rearrangement (translocation) or by a mutation in a gene other than UBE3A. These genetic changes can abnormally inactivate the UBE3A gene.
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Prader-Willi syndrome
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Prader-Willi syndrome is caused by the loss of active genes in a specific part of chromosome 15 (the 15q11-q13 region) in each cell. People normally inherit one copy of this chromosome from each parent. Some genes are active only on the copy inherited from a person's father (the paternal copy). This parent-specific gene activation is called genomic imprinting. Prader-Willi syndrome occurs when the paternal copy of chromosome 15 is partly or entirely missing. Researchers are working to identify genes on this chromosome whose absence is associated with the characteristic features of Prader-Willi syndrome.
In about 70 percent of cases, Prader-Willi syndrome occurs when the 15q11-q13 region of the paternal chromosome 15 is deleted in each cell. A person with a deletion in the paternal chromosome 15 will have no active genes in this region because the genes on the paternal copy are missing and the genes on the maternal copy are inactive.
In about 25 percent of cases, people with Prader-Willi syndrome inherit two copies of chromosome 15 from their mother instead of one copy from each parent. This phenomenon is called maternal uniparental disomy. A person with two maternal copies of chromosome 15 will have no active copies of these genes.
In a small percentage of cases, Prader-Willi syndrome is caused by a chromosomal rearrangement called a translocation. Rarely, the condition results from an abnormality in the DNA region that controls the activity of genes on the paternal chromosome 15.
- other chromosomal conditions
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A particular chromosomal change called an isodicentric chromosome 15 (previously called an inverted duplication 15) can affect growth and development. This small extra chromosome is made up of genetic material from chromosome 15 that has been abnormally duplicated. In some cases, the extra chromosome is very small and has no effect on a person's health. A larger isodicentric chromosome 15 can result in weak muscle tone (hypotonia), intellectual disability, seizures, and behavioral problems. This chromosomal change also has been found in some children with the features of autism or related developmental disorders affecting communication and social interaction.
Other changes in the number or structure of chromosome 15 can cause intellectual disability, delayed growth and development, hypotonia, and characteristic facial features. These changes include an extra copy of part of chromosome 15 in each cell (partial trisomy 15), a missing segment of the chromosome in each cell (partial monosomy 15), and a circular structure called ring chromosome 15. A ring chromosome occurs when both ends of a broken chromosome are reunited.
Is there a standard way to diagram chromosome 15?
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 15?
You may find the following resources about chromosome 15 helpful. These materials are written for the general public.
- Additional NIH Resources - National Institutes of Health
- Educational resources - Information pages
- 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 15?
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.
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