Length heterogeneity in a region of the human ribosomal gene spacer is not accompanied by extensive population polymorphism

We carried out a restriction enzyme analysis of human ribosomal DNA structure on total placental DNA using the Southern (1975) method. Studies on a single individual using HindIII, PstI, HpaI and BglII revealed that a region of the non-transcribed spacer near the 3′ end of the 28 S gene was heterogeneous in size. Four fragment classes were detected in this individual. Adjacent classes differed in size from one another by about 0·8 × 10³ bases. Analysis of 19 additional placental samples and four cell lines revealed no fragment classes other than those detected in the original sample. Mixing experiments carried out with all 20 placental DNA samples provided further evidence for the discrete nature of the population polymorphism in the length of this region of the spacer. This finding contrasts sharply with the almost continuous nature of the population polymorphism in the length of a region of the non-transcribed spacer in Xenopus laevis.