Hey! another brilliant question bridget!! Basically it is by a process called nondisjunction…which you may have heard of if you have done mitosis and meiosis in biology. Nondisjunction causes trisomy and monosomy (i.e. having three of a chromosome in a cell or only one copy).
Normally, when your cells undergo meiosis to become gametes (sex cells), chromosomes separate from each other in a process of disjunction so that each gamete will have half the number of chromosomes as a normal body cell. i.e. the cell splits into two and half the chromosomes go to one and half to the other.
However, when two chromosomes fail to separate, this event is called nondisjunction. This means that one cell ends up with both chromosomes, while the other cell has none.
When a sex cell that has two of the chromosomes fuses with a normal gamete, the result is that the zygote produced has three copies of that chromosome. This is called trisomy (“tri” means three). This is what happens in Down’s Syndrome with chromosome 21 and Edward’s with chromosome 18.
On the other hand, if the sex cell that has NONE of that chromosome, and fuses with a normal sex cell, the result is a zygote with only one copy of that chromosome- monosomy (“mono” means one).
Cri du chat and Turner’s syndrome are two conditions that arise from this!