RT Journal Article SR Electronic T1 Cytogenetic and molecular studies of trisomy 13. JF Journal of Medical Genetics JO J Med Genet FD BMJ Publishing Group Ltd SP 725 OP 732 DO 10.1136/jmg.24.12.725 VO 24 IS 12 A1 T Hassold A1 P A Jacobs A1 M Leppert A1 M Sheldon YR 1987 UL http://jmg.bmj.com/content/24/12/725.abstract AB Chromosome heteromorphisms, restriction fragment length polymorphisms, or both were used to study the parental origin of 33 cases of simple trisomy 13 and eight cases of translocation trisomy 13. The most common origin for the simple trisomies was non-disjunction at maternal meiosis I, while for the translocations an equal number of paternally and maternally derived cases was observed. In seven of the simple trisomies, information was obtained from both the cytogenetic and molecular markers, making it possible to study recombination between the two non-disjoined chromosomes. Five of the seven cases involved errors at meiosis I, with crossing over being detected in two of three cases of maternal origin and in one of two cases of paternal origin. This indicates that absence of recombination because of pairing failure is unlikely to be of major importance in the genesis of trisomy 13.