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.
Like Patau syndrome, Down syndrome is caused by having an extra copy of a certain chromosome. Individuals with Down syndrome usually have an extra copy of chromosome 21. Thus, Down syndrome is commonly called trisomy 21. Perhaps not surprisingly, the cause of the trisomy in both Patau and Down syndromes is similar. A phenomenon called nondisjunction is responsible for most cases of trisomy 13 and 21.