one can examine down syndrome patients and determine which
chromosomes 21 they have received using RFLP's. when this
was done in a recent study (sherman et al., 1994, human molec.
genetics 3:1529-35), it was determined that almost 90% were
the result of maternal nondisjunction (311/352), of which
75% occured at meiosis I (that is, both maternal homologues
were present in the trisomy 21 patient).
rebekah rasooly, st. john's university