Murthy, D SMurthy, S KPatel, J KTyagi, A A2009-05-282009-05-281989-10-01Murthy DS, Murthy SK, Patel JK, Tyagi AA. Nucleolar organizer region heteromorphism associated with trisomy-21: a risk factor for non-disjunction? Indian Journal of Experimental Biology. 1989 Oct; 27(10): 864-7http://imsear.searo.who.int/handle/123456789/62648An unusual nucleolar organizer region (double NOR) on chromosome 13 was observed in a Down syndrome child [47, XY, +21, dNOR(13)]. The variant chromosome was inherited from the mother [46, XX, dNOR(13)]. The extra chromosome 21 in the proband was maternal origin. The frequency of NOR chromosome association showed relatively high frequency in the mother and proband as compared to the controls. The result suggest that chromosome variants involving extra copies of NOR may indeed be involved in the meiotic nondisjunction of chromosome-21.engChild, PreschoolDown Syndrome --geneticsHumansMaleNondisjunction, GeneticNucleolus Organizer Region --ultrastructureRisk FactorsCase Reports