Structural anomalies of the X chromosome: personal observation and review of non‐mosaic cases

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Abstract
A new case is described of partial deletion of the long arm of the X chromosome, found in a 24-yr-old female with secondary amenorrhea; the karyotype of the proposita is 46,X,del(X)(q22). The previously published descriptions of non-mosaic structural anomalies of the X chromosome (X isochromosomes excepted) are reviewed with the goal of testing the recent hypotheses formulated about: the existence of an X inactivation center; the presence of a b segment remaining active on Xp the potential importance of a critical area on Xq linked to gonadal function the presence of normal gonadal function despite an Xp terminal deletion. The above-mentioned theories, as well as those concerning phylogenetic evolution of sex chromosome morphology presented by Lyon and Hoo receive support from practically all of the 149 cases compared. Regarding the features of the Turner syndrome, mapping of the X chromosome is proposed as follows: the genes involved in gonadal function seem to be located on the proximal part of Xp and on the distal part of Xq, whereas the genes whose absence is responsible for somatic features of the syndrome may be distributed along the length of Xp and the middle section of Xq(q21-q26). Some interesting analogies between the evolutionary model proposed by Hoo and the map visualized here are noted.