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.
Down syndrome (also called trisomy 21) is a chromosomal disorder caused by the presence of all or part of an extra 21st chromosome. Since it is cause by having a *third* set of certain genes, it is not dominant or recessive in the usual sense of the word.