KEYWORDSMosaic trisomy 1q; de-novo translocation 1/14; paternal origin of structural aberration; combination of meiotic and postzygotic aberration ABSTRACT Pure trisomies of the whole long arm of chromosome 1 are extremely rare and have been reported only once in association with mosaicism. We report on a malformed foetus with mosaic trisomy 1p11 to 1qter whose clinical features were partially in accordance with those of previously described trisomy 1q patients. An additional long arm of chromosome 1 was translocated onto 14p11.2 (karyotype: mos46,XY,der(14)t(1;14)(p11;p11.2)/ 46,XY). Mosaic formation of the partial trisomy 1 was investigated in seven different somatic tissues of first and second trimester pregnancy. The distribution of the pathologic cells was unequal, ranging from 4 to 93%. The duplicated region was paternal in origin. We were able to delineate two possible complex formation mechanisms involving paternal meiosis and postzygotic mitoses.