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.
The cause of Down syndrome is one of three types of abnormal cell division involving chromosome 21. All three abnormalities result in extra genetic material from chromosome 21, which is responsible for the characteristic features and developmental problems of Down syndrome.
Down syndrome is typically caused by what is called nondisjunction. Nondisjunction happens when a pair of chromosomes fails to separate during egg (or sperm) formation. When that egg unites with a normal sperm to form an embryo, the embryo ends up with three copies of chromosome 21 instead of the normal two.