Reviewed January 2009
What is chromosome 13?
Humans normally have 46 chromosomes in each cell, divided into 23 pairs. Two copies of chromosome 13, one copy inherited from each parent, form one of the pairs. Chromosome 13 spans about 114 million base pairs (the building blocks of DNA) and represents between 3.5 percent and 4 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 13 contains approximately 350 genes.
Genes on chromosome 13 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 13.
What chromosomal conditions are related to chromosome 13?
The following conditions are caused by changes in the structure or number of copies of chromosome 13.
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retinoblastoma
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A small percentage of retinoblastoma cases are caused by deletions in the region of chromosome 13 (13q14) containing the RB1 gene. Children with these chromosomal deletions may also have intellectual disability, slow growth, and characteristic facial features such as prominent eyebrows, a broad nasal bridge, a short nose, and ear abnormalities. Researchers have not determined which other genes are located in the deleted region, but a loss of several genes is likely responsible for these developmental problems.
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trisomy 13
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Trisomy 13 occurs when each cell in the body has three copies of chromosome 13 instead of the usual two copies. Trisomy 13 can also result from an extra copy of chromosome 13 in only some of the body's cells (mosaic trisomy 13).
In some cases, trisomy 13 occurs when part of chromosome 13 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 13, plus extra material from chromosome 13 attached to another chromosome. People with this genetic change are said to have translocation trisomy 13. The physical signs of translocation trisomy 13 may be different from those typically seen in trisomy 13 because only part of chromosome 13 is present in three copies.
Researchers believe that extra copies of some genes on chromosome 13 disrupt the course of normal development, causing the characteristic features of trisomy 13 and the increased risk of medical problems associated with this disorder.
- other chromosomal conditions
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Partial monosomy 13q is a rare chromosomal disorder that results when a piece of the long arm (q) of chromosome 13 is missing (monosomic). Infants born with partial monosomy 13q may exhibit low birth weight, malformations of the head and face (craniofacial region), skeletal abnormalities (especially of the hands and feet), and other physical abnormalities. Mental retardation is characteristic of this condition. The mortality rate during infancy is high among individuals born with this disorder. Almost all cases of partial monosomy 13q occur randomly for no apparent reason (sporadic).
Is there a standard way to diagram chromosome 13?
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 13?
You may find the following resources about chromosome 13 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 13?
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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