Several kinds of sexual mosaicism are seen in humans. The reason for the development of each of the following genotypes through nondisjunction at meiosis can be explained in the following way:
a) XX/XO
The genotype of XO is due to the process of nondisjunction which resulted in the loss of one X chromosome. This loss was after the single-celled zygote divided to form two cells.
b) XX/XXYY
Since XXY indicates the presence of the Klinefelter syndrome, this genotype could have occurred due to nondisjunction which would have led to Klinefelter syndrome. Then, after the single-celled zygote divided to form two cells, a nondisjunction event occurred in one cell for the Y chromosome. This results in the genotype of XX and XXYY.
c) XO/XXX
The genotype of XO indicates the absence of one chromosome while the genotype of XXX indicates the presence of an extra chromosome. Both these events are the result of nondisjunction. The nondisjunction of the X chromosome at the one-cell stage occurred which resulted in the given genotype.
d) XX/XY
The XX and XY zygotes fused to give this genotype. This could be due to fusion of zygotes of separate fertilizations. It could also be due to an egg fusing with a polar body of one X-sperm and one Y-sperm.
e) XO/XX/XXX
The genotype of XO indicates the absence of one chromosome while the genotype of XXX indicates the presence of an extra chromosome. Both these events are the result of nondisjunction. The nondisjunction of the X chromosome at the two-cell stage or later is responsible for the above genotype.