A complex chromosomal rearrangement with formation of a ring 4.

Ring chromosome formation, though uncommon, is now a well-recognized cause of congenital anomalies in man. On the basis of early work in other organisms, it is assumed that ring formation involves a break in each arm of the chromosome, with reunion to form a centric ring and an acentric fragment. The latter is subsequently lost, and the phenotype results essentially from deletion of this chromosomal material, complicated by duplications and deficiences arising in somatic cells due to the mitotic instability of ring chromosomes (McClintock, 1938).