Down syndrome and maternal age: the effect of erroneous assignment of parental origin.

It is tempting to assume that the maternal age effect in trisomy 21 is confined to cases arising from errors of maternal gametogenesis. However, it has been suggested that this hypothesis is incompatible with the results of studies, based on the subjective assessment of chromosome polymorphisms, of the parental origin of the additional chromosome. Contrary to the hypothesis, these studies appear to indicate that the ratio of maternal to paternal errors does not depend significantly on maternal age. I show here that the hypothesis need not be rejected if the proportion of published parental assignments that are incorrect is greater than or equal to 8%, a figure regarded as realistic by some experienced cytogeneticists.