Isochromosome for the short arm of X, a human 46, XXpi syndrome

SUMMARY Cytogeetical and clinical findings in a 17 year‐old female with te karyotype 46, XXpi are presented. The patient was 159 cm tall and had never menstruated. The gonads were not palpable and secondary sex characteristics were poorly developed. She had no somatic sgns of Turner's syndrome apart from a renal anomaly. The abnormal X chromosome could be identified as an Xpi by its characteristic fluorescence patterns and Giemsa staining properties after the ASG procedure. Autoradiography showed it to be the latest‐labelling chromosome in nearly all cells. Sex chromatin bodies were normal in number and decreased in size (79% of normal). These findings, taken with the incomplete data from two presumptive XXpi cases reported by others, indicate that the karotype 46, XXpi produces a clinical picture distinguishable from Turner's syndrome. The proposita and her father were Xg(a+), and her mother Xg(a−), indicating paternal derivation of the normal X chromosome. Hence the Xpi appears to be of maternal origin. Other interpretations are possible. The findings lend added support to the hupothesis that genes controlling gonodal development are carried in both Xp and Xq, whereas those affecting stature are in Xp but probably not in Xq. It is proposed that at isochromosome formation Xqi is far more likely to be produced than Xpi. This may be because breakage preferentially occcurs at the short‐arm end of the centromere region or in the short arm itself, as indicated by the existence of presumptive dicentric Xqi chromosomes.