Pericentromeric heteromorphism of human chromosome 18 as revealed by FISH-technique.

The pericentromeric heterochromatin contains tandemly repeated alphoid DNA sequences of about 171 bp in length. They are highly divergent from one chromosome to another due to chromosome specific alphoid subsets. In the present investigation, we used chromosome 18-specific centromeric probe (D18Z1) to evaluate the extent of pericentromeric heteromorphism classified by FISH-technique among 25 normal individuals. The hybridization signals were arbitrarily classified into five sizes when compared with the length of the short arm of chromosome 18. These are: negative (1), small (2), medium (3), large (4), and very large (5), with incidence of 0, 12, 24, 42, and 22 percent, respectively. Based on limited data, there were no chromosomes with negative signals while 42% of chromosome 18 had large-sized pericentromeric heterochromatin. The incidence observed earlier by restriction endonuclease Alu1 was different as compared to the present approach suggesting the complex heterogeneity of pericentric region of chromosome 18.