Balanced structural changes involving the human X: effect on sexual phenotype.

Reports of 107 cases (89 females and 18 males) with balanced X-autosome translocations and 11 cases with pericentric inversions (and their families) have been reviewed. Of the 78 informative females, 36 were infertile and had an X-breakpoint in the segment Xq13-26. Thus the existence of the critical segment has been amply confirmed by this review. However the finding of three normal fertile females with deletion of a part or the whole of this segment suggests that it is not the break in the critical segment, as was first thought, but rather the contact between loci in the critical segment and loci external to it that is responsible for the abnormal sexual phenotype. Six females were exceptions to the critical region hypothesis. The X-breakpoints in these were in the critical region and yet each had had one or more children. Three of the six women (two inversions and one translocation carriers) were not wholly normal as they suffered from menstrual cycle disorder, secondary sterility and/or early menopause. Four of the six (including the three true exceptions) had a breakpoint in band Xq22. It is possible that there are two critical segments within Xq13-26 separated by a small segment in band Xq22. Seven of the nine adult male carriers of the balanced X-autosome translocations were infertile. Even from this limited number of cases it appears that male carriers of X-autosome translocations are likely to suffer from a disturbance of spermatogenesis as is the case in mouse and drosophila. Male carriers of X-inversions do not necessarily suffer from infertility. Results from one family with X-inversion suggest that some sort of a position effect in the X affecting the sexual phenotype may also be operating in the male.