Karyotypes of six members of a family in three generations consistently exhibited an additional minute centric chromosome. Except for the mentally retarded propositus, the remainder of the family proved to be clinically normal. The early replicating fragment was frequently found to be associated with acrocentric chromosomes and was located peripherally in the majority of metaphase spreads. As compared to similar cases reported by various authors, the chromosomal minute is considered to be genetically inert, and coincidence with certain clinical features is likely to be fortuitous. It is suggested that the fragment represents one counterpart of a centric fusion event, occurring more often in the general population than has been previously assumed owing to lack of phenotypic expression in carrier persons.
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