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.
Causes of Down Syndrome. From what is known about the causes of Down syndrome, environmental factors and parents' behavioral activity do not seem to play a role. Most cases (approximately 92 percent) are caused by trisomy 21, which means each cell in the body has three copies of chromosome 21 instead of the usual two copies.