Population studies of INV(9) chromosomes in 4,300 Japanese: incidence, sex difference and clinical significance.

Population incidence of a chromosome 9 with an inversion of qh, inv(9), was surveyed using C-banding in a total of 4,367 Japanese which consisted of five patient groups and a normal control group. The inv(9) incidence was 1.65% in the normal control group (n = 1,513) and 1.52% in the Down syndrome patient group (n = 1,246). The incidence of female carriers was 1.7 times higher than that of male carriers in the above two groups. The sex difference was significant (p < 0.05). Moreover, the incidence in the 47,XXY male group (n = 277) was slightly higher than female carrier incidences. The excess of carriers about 2 times was observed in the habitual abortion couple group (n = 694) and the spontaneous abortus group (n = 181), both having normal karyotypes. The present data probably indicate the existence of a genetic effect of inv(9) on human reproductive performance, while they could not explain the sex difference found in inv(9) carriers. Our speculation for the sex difference has been presented.