Trisomy Definition:

A few people only who know that's trisomy is a chromosomal condition characterised by an
additional chromosome.

A trisomy is a genetic disease that results from the formation of a third excess copy of chromosome No. 13, which leads to the appearance of birth defects in the heart and the central nervous system and a retardation in the mental capabilities of the child accompanied by obstruction or delay in the normal process of development, and among the most prominent signs: diaphragm (microcephaly); Pre-amalgamation), defects in the structures of the eyes including the small eyeball, the deformation of Peters, the cataract (cataract), the indentation, the dysplasia of the retina, the separation of the ark, the separation of the lung, the alum and the ark Skeletal and other physical structures such as hyperdactylia (polydactyly) and calcaneal Toxoplasma, secrecy and hematomas, and aplastic rashes, cleft palatine, and genital anomalies, defects renal, and ventricular septal defect of the heart, the only umbilical artery, occurs by one per ten thousand live births, and the so-called Also the Patau syndrome, the D-triangle syndrome (D).
 A person with a trisomy has 47 chromosomes instead of 46.
 Those are most trisomy forms ,  "Down syndrome", " Edward syndrome" and "Patau syndrome"

What causes trisomy?