الفهرس 
IntroductionOnly 14 pages are availabe for public view
 |  | from | 133 |  |  |
| | | from | 133 | | |
Abstract
Down syndrome is the most autosomal aberration occurring in approximately 1 in 600 live births. The incidence however increases significantly with advancing maternal age. In women under 30, the incidence is about 0.6 in 1000 births, this increase to approximately 20 in 1000 births for mothers aged 45 and over.
Down syndrome is the result of a trisomy of chromosome 21 or the translocation of q22 segment to another chromosome. This has led some investigators to conclude that q22 portion is responsible for this disease.
Two enzymes involved in purine metabolism, gars and airs have been mapped to chromosome 21. as altered purine metabolism is related to mental retardation, this could explain the lowered intellectual capacity of these patients.