A rare de novo duplication of chromosome 21q22.12→q22.3 associated with Down syndrome – A case report

Genetics of Down syndrome (DS): (A) The classical trisomy 21 is caused by a meiotic nondisjunction event. (B) Mosaicism , used to describe the presence of more than one type of cell in a person, is usually described as a percentage. Mosaic DS occurs when some of the cells in the body are normal, and some cells have trisomy 21. This is the cause of 1–2% of the observed DS. There is evidence that mosaic DS may cause less developmental delay, on average, than full trisomy 21. (C) Translocation DS : The extra chromosome 21 material that causes DS may be due to a Robertsonian translocation. The long arm of chromosome 21 is attached to the long arm of another chromosome or itself (isochromosome). Translocation Down syndrome can be de novo; that is, such a thing is not inherited, but may be occurring at the time of an individual's conception, or may be inherited from a parent with a balanced translocation. It is the cause of about 4–5% of the observed DS. It does not show the maternal age effect, and is just as likely to have come from any of the parents. (D) Duplication DS : A region of chromosome 21 may undergo a duplication event leading to extra copies of some of the genes on chromosome 21.