Ring chromosome

A ring chromosome is an aberrant chromosome whose ends have fused together to form a ring. Early observations of ring chromosomes were made during the development of cytogenetics in the early 20th century. Lilian Vaughan Morgan reported an unusual X chromosome in Drosophila melanogaster in 1926 that appeared “almost or entirely closed and somewhat rounded,” a structure later interpreted as a ring chromosome. In 1930, Mikhail Sergeevich Navashin described chromosomes in species of Crepis whose ends had fused to form a closed ring, providing the first explicit cytological description of ring chromosomes in plants. Barbara McClintock subsequently reported ring chromosomes in maize in 1931 and carried out detailed studies of their behavior and instability in plant cells. Early cytological syntheses subsequently listed ring chromosomes in Crepis (Navashin, 1930), Drosophila (L. V. Morgan, 1933), and maize (McClintock, 1932).

A ring chromosome is denoted by the symbol r in human genetics and R in Drosophila genetics. Ring chromosomes may form in cells following genetic damage by mutagens like radiation, but they may also arise spontaneously during development.