A review of case reports on patients with ring chromosome revealed 30 individuals (plus two fetuses) who inherited the ring from a total of 23 carrier parents (21 mothers and 2 fathers). The proportion of cases with inherited rings, among all patients with a ring, was calculated to be 5.6% as an upper limit. However, because of a propable difference in survival and fertility between individuals with transmitted and de novo rings, and because of the preferential publication of cases involving inherited rings (and thus a publication bias), the proportion of inherited rings should in reality be no more than 1%. Out of 30 transmitted rings, there were 9 where parent and child were both mosaics, suggesting an inherited instability of the chromosome involved leading to de novo re-formation of the ring in the second generation. The relatively mild clinical manifestations of ring chromosomes, in general, was found to be even more striking in familial cases. In half of the offspring the phenotype was very similar to that of the parent. However, in about a third of cases the offspring were more severely (mentally) affected. This fact should be considered in genetic counseling of clinically normal women who carry a ring chromosome.
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