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.
Research shows that three types of chromosomal changes can lead to Down syndrome. 1  Complete trisomy 21. 2  In this case, an error during the formation of the egg or the sperm results in either one having an extra chromosome. 3  So after the egg and sperm unite, the resulting cells will also have three copies of chromosome 21. The complete extra copy of chromosome 21 is in all of the person's cellsâor a complete trisomy. Complete trisomy 21 is the cause of about 95% of Down syndrome cases. Mosaic trisomy 21.