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.
Trisomy 21â More than 90% of Down syndrome cases are caused by trisomy 21. An extra chromosome (chromosome 21) originates in the development of either the sperm or the egg. When the egg and the sperm unite to form the fertilized egg, three (rather than two) chromosomes 21 are present.
(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.