For all DS cases routine G- banding cytogenetic analysis was done and extended mosaic study was performed when indicated.

Table (2) presents the cytogenetic data of the studied group.

Karyotype ... No. of cases ... Percentage

47,XY,+21 ... 522 ...

93.98

47,XX,+21 ... 442 ...

46,XY,der (13;21) (q 10;q 10),+21 ... 2 ...

3.5

46,XY,der (14;21) (q 10;q 10),+21 ... 8 ...

46,XX,der (14;21) (q 10;q 10),+21 ... 7 ...

46,XY,der (15;21) (q 10;q 10),+21 ... 1 ...

46,XY,der (21;21) (q 10;q 10),+21 ... 13 ...

46,XX,der (21;21) (q 10;q 10),+21 ... 6 ...

47,XX,+21 or 47,XY,+21/ 46,XX or 46,XY ... 22 ... 1.84

Non-Classical Down syndrome ... 7 ... 0.68

Total ... 1030 ... 100

Table (2): Cytogenetic data of the DS cases

Translocation was present in 3.5 % of cases. All types of Robertsonian translocation were detected except t (21;22). Most of the cases had t (21; 21), which was present in 51.35 % of cases, followed by t (14; 21), which was present in 40.5 % of cases, t (13; 21) in 5.4 %, and t (15; 21) in 2.7 % of cases of translocation.

Non-classical karyotypes were present in 7 patients (0.68%), and included:

Non-classical karyotypes with sex chromosome abnormality:

Down syndrome and Turner syndrome:

46,X, +21 [5] / 46,XX [70]

46,XX der (21; 21)(q 10; q 10), +21[11] / 45,X [9]

Down syndrome and Klienfelter syndrome:

- 47,XXY, der (21; 21)(q 10; q 10), +21

Non-classical karyotypes with marker chromosome:

48,XY, + 21, + mar

47, XX, +21 [13] / 47, XX, +mar [6] / 46, XX [6]

47,XY, +21, t (4; 21) (q 31.1; q 22)

46,XY, der (21; 21)(q 10; q 10), +21 [16] / 46,XY [49]