Ring chromosome

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. Actually, in rare cases, the telomeres at the ends of a chromosome fuses without any disappearing of material.

Associated diseases
Human genetic disorders can be caused by spontaneous ring chromosome formation; although ring chromosomes are very rare, they have been found in all human chromosomes. Disorders arising from the formation of a ring chromosome include ring chromosome 20 syndrome where a ring formed by one copy of chromosome 20 is associated with epilepsy; ring chromosome 14 and ring chromosome 13 syndrome are associated with mental retardation and dysmorphic facial features; ring chromosome 15 is associated with mental retardation, dwarfism and microcephaly. Ring formation of an X-chromosome causes Turner syndrome.

Reference

 * Schmidt et al. 1981. Ring chromosome 14: a distinct clinical entity. Journal of Medical Genetics 18:304-307