This abnormality in human beings was first reported by down and later studied by Lejuene, Gautier and Turpin in 1959. Also known as Mongoloid idiocy, this syndrome is characterized by mental retardation and other clinical symptoms.
Some of the other morphological manifestations of this abnormality are- mongoloid facial appearance, webbed neck, a flat nasal bridge, presence of epicanthic fold (small fold of the skin over the inner corner of the eye as seen in Mongoloid race), brush field spots around the Iris, short fingers and a gap between the first and second toe.
The most striking feature of this syndrome however is mental retardation. Individuals suffering from Down’s syndrome will have an IQ between 25-50 as against the normal IQ of 80. Dermatoglyphic pattern (lines on the palm) shows a simian (as in monkeys) crease. Frequently all 10 fingers have ulnar loops.
Down’s syndrome is a result of abnormality in chromosome no. 21. In this pair, instead of there being two homologus chromosomes, there will be three chromosomes. This condition is known as trisomy. Individuals with Down’s syndrome will have 47 chromosomes instead of the normal 46.
This abnormality results due to non separation of the duplicated chromosome during meiosis. As a result, some eggs will have 24 chromosomes instead of 23. If these are fertilized by sperms (which have normal 23 chromosomes) the re- sultant-zygote will have 47 and the individual will show Down’s syndrome.
A detailed clinical study of Down’s syndrome has been made in Western countries. It has been found out that trisomy of chromosome 21 occurs in one in 700 births and it is the most common of autosomal trisomies.
There is a relationship between the increased age of the mother and the incidence of Down’s syndrome among the children. Down’s syndrome individuals live for varying number of years. A Down’s male is usually sterile, while females rarely produce children.
There is no cure for Down’s syndrome as it is a genetic disorder.