Karyological characteristics of Down's syndrome: clinical and theoretical aspects

These data have been collected from St. Petersburg Down Syndrome Register that comprises information on 1778 liveborn children with the Down syndrome, including three twin sets, ascertained within 1970-1996. Karyotypes were obtained in 1223 cases, of which 1119 (90.7%) displayed regular trisomy. Mosaicism was found in 44 cases (3.6%), including 21 males and 24 females, and among these one familial case of mosaicism in a daughter and in a healthy mother. Of 70 cases of translocations, 41(5.7%) were Robertsonian D ones. 21 (17 inherited, 16 de novo and 8 of unknown origin), 28 translocations of isochromosomes 21q; 21q (1 inherited translocation 21; 22, 22 de novo and 5 of unknown origin). One child received the anomaly from his 46XX/45XX, t(D;G) mother-carrier. In 6 cases, free trisomy 21 was associated with structural or numerical anomalies: 46XY,t(13;14)mat + 21 in twins, 47XY,t(C;C) + 21, 47XY,t(10;15)pat + 21, 47XY,inv(19)mat + 21, 47XX + 21/48XX + 21 + ring, 48XXX + 21. In 12 families parental mosaicism was shown or suspected. In 6 families one parent had chromosome anomaly, in three cases it was not inherited: t(15;22) and t(6;21) in mothers and an additional small marker in a father. In cases confirmed cytogenetically an increased sex ratio was shown (679 males and 551 females, SR = 1.23), but it was not shown in patients not tested cytogenetically (264 males and 275 females, SR = 0.96, different from the expected 297 males and 242 females, P < 0.01).