Another type of genetic defect is caused by errors in chromosome copying during the cell division which produces the gametes (meiosis). Cells divide and copy themselves to produce gametes. Sometimes, errors happen in the way that chromosomes are copied during this process. These errors are then copied again and again as cells keep dividing and copying themselves. The most common chromosomal disorder is Down syndrome ('Mongolism'). This condition happens when a child has three copies of the 21st chromosome, instead of two.