A 5 year old male patient complains of swollen and painful gums. He gives a history of delayed eruption of teeth. On extra-oral examination, an underdeveloped midface and prognathic jaw relationship was seen. Intraoral finding revealed open bite, macroglossia and generalized periodontitis. Which of the following is the most common etiological factor for this condition?

Mosaicism is an abnormality defined as the presence of 2 or more cell lines in a single individual. Polyploidy is a common abnormality seen in 1st trimester pregnancy losses. Monosomies occur when only 1, instead of the normal 2, of a given chromosome is present in an otherwise diploid cell. In approximately 95% of the cases of Down syndrome, there are 3 copies of chromosome 21. The origin of the supernumerary chromosome 21 is maternal in 97% of the cases as a result of errors in meiosis. The majority of these occur in maternal meiosis I (90%). Approximately 1% of persons with trisomy 21 are mosaics, with some cells having 46 chromosomes, and another 4% have a translocation that involves chromosome 21.