Variation in Alphoid Dna Size and Trisomy 21: A Possible Cause of Nondisjunction

The cause of nondisjunction of chromosome 21 remains largely unknown. In the present report, we investigate the hypothesis that variation in alphoid DNA size has a role in trisomy formation. Pulsed-field gel electrophoresis was used to examine the chromosome 21 alphoid DNA array lengths in 23 families (all of Northern European ancestry) with an affected child with trisomy 21 in whom the parental and meiotic origin of nondisjunction had been determined as maternal meiosis I, and in 38 controls. Initially, the combined alphoid size of both chromosome 21 homologues was assessed. This indicated an association between small combined alphoid size and maternal meiosis I nondisjunction. Moreover, in a subset of the families under study (n=12), it was possible to study the α21-I size of individual chromosome 21 homologues (simple alphoid size); this provided further evidence that the risk for nondisjunction is related to the size of the alphoid array of one of the two chromosome 21 homologues being small.