CENTROMERIC ALPHOID DNA HETEROMORPHISMS OF CHROMOSOME-21 REVEALED BY FISH-TECHNIQUE

The centromeric heterochromatin of chromosome 21 has been evaluated by the fluorescence in situ hybridization (FISH) technique. It was found that the alphoid DNA sequences of pericentromeric regions of chromoso me 21 were highly heteromorphic when a centromeric specific probe was hybridized to these sequences. The variations were so extreme that the y could even be arbitrarily classified into at least five sizes by com parison with the length of the short arm (p) of chromosome 18. They ar e: negative (1); small (2); medium (3); large (4); and very large (5). We used 15 normal cases and 12 individuals with trisomy 21 (Down synd rome), and the incidences for these five classes were 3.0%, 22.7%, 59. 2%, 13.6% and 1.5%, respectively. At least 3% of the chromosomes no. 2 1 did not show any trace of hybridization signals, which apparently es cape detection at interphase level as well. Although, the variations o bserved in the present study are continuous, the proposed classificati on may yield some implications for future investigations.