Non-separation of chromosomes (chromosomal dysjunction) results in abnormal mitosis. It usually occurs immediately after zygote formation and results in mosaicism.

In mosaicism, the number of chromosomes is not the same in all the cells of the body. For example, nearly 50% of the body cell population may have a certain number of chromosomes while the remaining 50% may have a different number of chromosomes (mosaic = designs by inlaying small bits of colored stone;, glass, tile, etc. in mortar).

If half the cells in the body are 44 + X Y and the remaining cells are 44 + XXY, the mosaicism is denoted symbolically as XY/XXY.