1. Lejeune, J., Gautier, M. & Turpin, R. Etudes des chromosomes somatiques de neuf enfants mongoliens. C. R. Aca. Sci. Paris 248, 1721–1722 (1959).
2. Hook, E.B. In Trisomy 21: Research perspectives (eds deLa Cruz F.F. & Gerald P.S.) 11–88 (University Park Press, Baltimore, 1981).
3. Penrose, L.S. The relative effects of paternal and maternal age in mongolism. J. Genet. 44, 17–28 (1933).
4. Hassold, T. & Chiu, D. Maternal age-specific rates of numerical chromosomal abnormalities with special reference to trisomy. Hum. Genet. 70, 11–17 (1985).
5. Hassold, T.J. & Jacobs, P.A. Trisomy in man. Ann. Rev. Genet. 18, 69–97 (1984).