Fluorescence in situ hybridization (FISH) with an alphoid chromosome 21-specific probe was used for analy­sis of centromeric heterochromatin variations in 36 fami­lies with Down’s syndrome (DS) offspring. Variants were arbitrarily classified into five classes by size, estimated by comparison of the length of the short arm (p) of chromo­some 18. Segregation of alphoid DNA variants allowed investigation of the parental and meiotic origin of non-disjunction in 55.5% (20/36) of the families. Parental and meiotic origin of non-disjunction using a cytogenetic tech­nique was detected in 75% (27/36) of the families.

We have determined paternal origin in six families and maternal origin in 14 families by FISH, and eight families with paternal origin and 19 families with mater­nal origin of chromosome 21 by the cytogenetic technique. Correlation between non-disjunction of chromosome 21 and centromeric alphoid DNA variants have not been found. Therefore, the risk of nondisjunction is not related to the size of the alphoid DNA block of chromosome 21.

Key words: Alphoid DNA Non-disjunction; Down’s syndrome (DS); Fluorescence in situ hybridization (FISH); Chromosome 21.