Embryonic Development of KS
Klinefelter syndrome occurs in the embryo in a process error called nondisjunction. Nondisjunction is when chromosomes fail to separate in these vital stages of development and will normally result in the gain or loss of a chromosome. This process can be seen below:
As you can see the diagram clearly represents the cells being incorrectly replicated and results in an additional X (pink line) chromosome being formed. The diagram below further shows the process of nondisjunction replication in meiosis 1 and 2.
The diagram below shows the proper disjunction and nondisjunction in anaphase. Note how both chromosomes are off to one side in nondisjunction rather than evenly on both sides.
Nondisjunction can occur in both meiosis I or II in the development and replication of cells, but will make no difference to the severity of the syndrome. See below for visualisation:
The video below shows the replication process of the sex chromosomes (X, Y) in the body: