After fertilisation of a normal egg nondisjunction may occur
during a mitotic division in the developing embryo giving rise
to daughter cells that are trisomic and nulisomic for the
chromosome involved in the disjunction error. The nulisomic
cell would not be viable, but further cell division of the trisomic
cell, along with those of the normal cells, leads to chromosomal
mosaicism in the fetus. Alternatively a chromosome may be lost
from a cell in an embryo that was trisomic for that
chromosome at conception. Further division of this cell would
lead to a population of cells with a normal karyotype, again
resulting in mosaicism.
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