Lejune and co workers (1959) were the first to report the presence of an extra chromosome 21(trisomy 21) in the cells of DS individuals - a landmark in the field of congenital disorders in general and, in the area of cytogenetics in particular.

Over 90% of the DS cases result from classical trisomy 21 while the remaining 10% may exhibit mosaicism or tanslocations (rearrangements) involving chromosomes 14 and 21.

Down syndrome is the most common genetic cause of mental retardation and is also credited to be the most common chromosomal abnormality among new borns. The condition occurs as one in every 600-800 live births.

IQ is known to vary between 30-50.

It has been widely reported that DS children are more commonly born to older mothers - but in a country like India too, where women get married and produce children at an early age, this condition is equally prevalent as anywhere else in the world.

Couples who already have a DS child have increased chances of recurrence compared to the general population.

DS children are immuno-deficient and, therefore, it was common for them to die at infancy as they are easily susceptible to infectious diseases. But in the recent years, with the development of life saving drugs, death among DS population is on the decline.

Down syndrome predisposes the affected to certain cancers. These individuals are at a 20-fold increased risk to develop leukemia in comparison to other individuals. It has been proposed that altered function of genes on chromosome 21 leads to altered hemopoiesis thereby predisposing the trisomic children to develop leukemia.

Cardiovascular deaths appear in 40% of DS individuals contributing to the increased number of their deaths in infancy.

Gastrointestinal malformations and congenital hematological abnormalities are also more common in them.

Fertility is uncommon in DS males (2 cases reported thus far), while pregnancies among females have been reported.

DS is a genetic disorder and therefore incurable. However the clinical complications arising from this condition can be efficiently managed.