I. Introduction. 1  The petroleum industry began with the successful drilling of the first commercial oil well in 1859, and the opening of the first refinery two years later to process the crude into kerosene.
The cause of Down syndrome is one of three types of abnormal cell division involving chromosome 21. All three abnormalities result in extra genetic material from chromosome 21, which is responsible for the characteristic features and developmental problems of Down syndrome.
(Click Trisomy 21 Genetics for more information about trisomy 21.) The cause of Down syndrome in 2 to 4 percent of cases occurs when only some of the body's cells have an extra copy of chromosome 21. These cases are called mosaic Down syndrome.