A CHROMOSOMAL aberration that has not been previously reported was encountered during study of an 8-year-old Caucasian girl with multiple developmental anomalies and mental retardation. Her chromosome complement included an extra small metacentric chromosome with satellites on both ends, presumably an autosomal isochromosome. Isochromosomes have heretofore been described for the long arm of the X chromosome1-3 and more recently for the long arm of a 17-18 chromosome.4 Small metacentric chromosomes5,6 have been reported by several authors, and these may represent short arm isochromosomes. The D/D7 and G/G8,9 translocations may be alternately interpreted as isochromosomes for the long arm of one of the D (13-15) and G (21-22) chromosomes, respectively.
It is well recognized that excess genetic material may exist in viable human cells and that this extra material may be associated with various stigmata. At present, there are only three established autosomal trisomies in man: