ALTHOUGH it had been suspected for many years that specific clinical syndromes, characterized by multiple congenital anomalies, might be associated with specific chromosome abnormalities, it was not until 19561 that techniques became available to study human chromosomes in detail.
It has been determined that man has 46 chromosomes, 22 pairs of autosomal chromosomes and one pair of sex chromosomes. It is also possible to observe various structural abnormalities as well as an increase or decrease in the total number of chromosomes.
The 22 pairs of autosomal chromosomes are numbered 1 through 22, according to the Denver classification, and grouped in seven categories, A through G, according to the size of the chromosome and site of the centromere.
Three autosomal trisomy syndromes have been firmly established. In these cases an extra chromosome has been added to one of the pairs. These established trisomy syndromes are trisomy 21 or 22 (mongolism