Cytogenetic profile of Down syndrome in Alexandria, Egypt.

During 1992-2001, 673 Down syndrome patients were referred to the Department of Human Genetics in Alexandria. Regular (free) trisomy 21 constituted 95.4% of cases; Robertsonian translocation 2.7%; and mosaicism 0.7%. In 8 cases, regular trisomy 21 was associated with structural or numerical chromosome anomalies. Translocation was parentally inherited for 33.3% of cases and maternal transmission was twice as common as paternal. Two translocated Down syndrome fetuses were diagnosed prenatally in a t(14;21) carrier mother. Mean maternal age was high in regular trisomy 21 (38.2 years) but not in translocation (25.3 years). There was an excess of males in all groups except the mosaic group where the male:female ratio was 0.67. Cytogenetic investigations assist in patient management and family counselling.