Jérome Lejeune

During the mid-twentieth century and more precisely in 1950 were undertaken studies and research on the karyotype and were refined techniques for chromosome analysis.
In 1958, Jerome Lejeune discovered that Down syndrome was the result of the presence of a chromosome in excess in the Chromosome 21, thus at the name of Down's Syndrome, began to be used as an alternative, the definition of Trisomy 21.
The experimental research that led to this discovery was conducted by a French scientist, Marthe Gautier, co-author of the paper describing the presence of this chromosome in excess.
In people with Down syndrome, the chromosome in excess involves the presence of greater amounts of genetic material.
This implies a greater expression of 50% of the more than 300 genes located on chromosome 21.
Some research seems to have been able to identify where in the chromosome, is situated the critical regiom for Down syndrome, also they revealed the causes of various problems such as:
Cognitive delay.
Alteration of oxygen metabolism.
Most protein oxidation resulting in DNA damage and premature aging.
Lack of lymphocytes and less antibody resulting in a greater vulnerability to infection.

As regards the details of these studies are very technical, please refer to the following article.