Our study ... Palliam and Huether, 1986 ... ...

5.4 ... 0 ... t (13;21)

40.5 ... 45.7 ... t (14;21)

2.7 ... 2.9 ... t (15;21)

51.35 ... 40 ... t (21;21)

0 ... 2.9 ... t (21;22)

Table (10): percentage of translocation cases in our study and that of Palliam and Huether, 1986 (31).

It is evident from the previous tables that we have more case of non- classical karyotype and t (21; 21) in our population. The origin of denovo t (21; 21) DS are usually different from the other reciprocal translocation. For the majority of such chromosomes are not centric fusion or whole arm exchange chromosomes, rather they are isochromosomes (iso 21 q) resulting from fusion of sister chromatides (32,33). Whether isochromosomes arise during oogenesis is not established, and it is argued that isochromosomes arise post conception in somatic tissues of trisomic conceptus (34) . The relation of this theory to our findings could not be explained and needs further research.

We concluded that, in Egypt with 1.6 million births / year and estimated risk of 2285 DS births annually, the concept of preventive genetics should be reinforced with a national policy targeting both health professionals and general publics to offer prenatal genetic screening for all pregnant ladies and prenatal diagnosis for screen positive cases. This needs an integrated system including proper diagnostic facilities, trained personnel and professional staff.

References:

DeGrouchy J and Turleau C. Clinical atlas of human chromosomes. Wiley medical publication. John Wiley and sons. New York, Toronto. 2 nd ed. 1983. pp 70.