A ring chromosome is a chromosome whose arms have fused together to form a ring. A ring chromosome is denoted by the symbol r. Ring chromosomes may form in cells following genetic damage by mutagens like radiation, they may also arise spontaneously during development.

Formation

Normally, the ends of a chromosome are lost, enabling the arms to fuse together. However, ring formation can also occur with only one end being lost. In rare cases, the telomeres at the ends of a chromosome fuse without any disappearing of material.

Associated diseases

Human genetic disorder

Genetic disorder

A genetic disorder is an illness caused by abnormalities in genes or chromosomes, especially a condition that is present from before birth. Most genetic disorders are quite rare and affect one person in every several thousands or millions....

s can be caused by spontaneous ring chromosome formation; although ring chromosomes are very rare, they have been found in nearly all human chromosomes. Disorders arising from the formation of a ring chromosome include ring chromosome 20 syndrome

Ring chromosome 20 syndrome

Ring chromosome 20, ring-shaped chromosome 20 or r syndrome is a rare human chromosome abnormality where the two arms of chromosome 20 fuse to form a ring chromosome. The syndrome is associated with epileptic seizures, behaviour disorders and mental retardation.When only one copy of chromosome 20...

 where a ring formed by one copy of chromosome 20 is associated with epilepsy

Epilepsy

Epilepsy is a common chronic neurological disorder characterized by seizures. These seizures are transient signs and/or symptoms of abnormal, excessive or hypersynchronous neuronal activity in the brain.About 50 million people worldwide have epilepsy, and nearly two out of every three new cases...

; ring chromosome 14 and ring chromosome 13 syndrome are associated with mental retardation

Mental retardation

Mental retardation is a generalized disorder appearing before adulthood, characterized by significantly impaired cognitive functioning and deficits in two or more adaptive behaviors...

 and dysmorphic facial features; ring chromosome 15 is associated with mental retardation, dwarfism

Dwarfism

Dwarfism is short stature resulting from a medical condition. It is sometimes defined as an adult height of less than 4 feet 10 inches  , although this definition is problematic because short stature in itself is not a disorder....

 and microcephaly

Microcephaly

Microcephaly is a neurodevelopmental disorder in which the circumference of the head is more than two standard deviations smaller than average for the person's age and sex. Microcephaly may be congenital or it may develop in the first few years of life...

. Ring formation of an X-chromosome causes Turner syndrome

Turner syndrome

Turner syndrome or Ullrich-Turner syndrome encompasses several conditions in human females, of which monosomy X is most common. It is a chromosomal abnormality in which all or part of one of the sex chromosomes is absent...

. Symptoms seen in patients carrying ring chromosomes are more likely to be caused by the deletion of genes in the telomeric regions of affected chromosomes, rather than by the formation of a ring structure itself.

Complex rearrangements including segmental microdeletions and microduplications have been seen in numerous ring chromosomes providing important clues regarding the mechanisms of their formation.

External links

• Atlas of Genetics and Cytogenetics in Oncology and Haematology, explains HOW the ring formation causes abnormalities.