Origin of trisomy 21 in Down syndrome cases from a Spanish population registry

We have carried out a population-based study on the origin of the extra chr omosome 21 in 38 Families with Down syndrome (DS) offspring in El Valles (S pain). From 1991 to 1994, a higher prevalence of DS (22.7/10 000 live birth s, stillbirths and induced abortions) was found compared to the majority of EUROCAT registries. The distribution of trisomy 21 by origin was 88 % mate rnal (90.6 % meiosis I, 6.2 % meiosis II, 3.1 % maternal mosaicism), 5.6 % paternal (50 % meiosis I, 50 % meiosis II) and 5.6 % mitotic. The percentag e of parental mosaicism was 2.7 %. These percentages are similar to those p reviously reported. Recombination study revealed a maternal meiosis I genet ic map of 32.68 cM (approximately one-half the length of the normal female map). Mean maternal age among non-recombinant cases involving MI errors was significantly lower (31.1 years) than among those cases showing one observ able crossover (36.1 years) (P < 0.05); this could support the hypothesis t hat 'achiasmate' chromosomes may be subject to aberrant segregation regardl ess of maternal age. (C) 2000 Editions scientifiques et medicales Elsevier SAS.