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

We describe a new case of partial deletion of the long arm of the X chromosome, found in a 24-year-old female with secondary amenorrhea; the karyotype of the proposita is 46,X,del(X)(q22). We take this opportunity to review the previously published descriptions of non-mosaic structural anomalies of the X chromosome (X isochromosomes excepted) with the goal of "testing" the recent hypothesis formulated about: (a) the existence of an X inactivation center (Therman et al. 1974b); (b) the presence of a "b" segment remaining active on Xp (Therman et al. 1976); (c) the potential importance of a critical area on Xq linked to gonadal function (Sarto et al. 1973); and (d) the presence of normal gonadal function despite and Xp terminal deletion (Fraccaro et al. 1977). We conclude that the above-mentioned theories, as well as those concerning phylogenetic evolution of sex chromosome morphology presented by Lyon (1974) and Hoo (1975), receive support from practically all of the 149 cases we compared. Regarding the features of the Turner syndrome, we propose "mapping" of the X chromosome 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). Furthermore, we note some interesting analogies between the evolutional model proposed by Hoo (1975) and the map we visualize.