The risk for recurrence of non disjunction trisomy 21 is conventionally considered
to be less than 1%. Within a span of 3 years, we observed recurrence of non disjunction
trisomies in four families. The objective of the present study was to determine low
level mosaicism in either of the couple and to identify the parental origin of additional
chromosome 21/18. The four couples who had recurrent trisomic conceptions were investigated
for the underlying mosaicism by analysis of 100 cells from peripheral blood of the
couple and the parental origin of supernumerary chromosome 21/18 were identified using
microsatellite markers. Low level mosaicisms in peripheral lymphocytes of couple were
ruled out for all four families. Microsatellite markers have shown maternal origin
of chromosomal nondisjunction for all the families and defective first meiotic division
as the most common mechanism for nondisjunction. This observation raises the need
for discussing the option of invasive testing while counseling the couple with an
affected child with non-disjunction trisomy as the risk for recurrence of trisomies
in subsequent pregnancy might not be as low as 1%.
Keywords
Nondisjunction - Trisomy 21 - Trisomy 18 - Recurrence - Meiosis I - Meiosis II
