Minisatellite variant repeat (MVR) mapping: analysis of 'null' repeat units at D1S8.
Minisatellite variant repeat mapping by PCR (MVR-PCR) is a new approach to studying variation in human DNA which analyses interspersion patterns of variant repeats within minisatellite arrays. MVR-PCR has been applied to the hypervariable human minisatellite D1S8 which contains two major classes of variant 29bp repeat units designated a-type and t-type. The MVR-PCR assay uses a- or t-type specific primers, together with an amplimer at a fixed site in the DNA flanking the minisatellite, to reveal the interspersion patterns of variant repeats along an allele. Extreme levels of variation are seen both in the internal structures of individual alleles and in the digital code generated from the two superimposed alleles in total genomic DNA. However, occasional repeat units fail to amplify in MVR-PCR, signifying the existence of further repeat sequence variants termed 'null' or O-type repeats. Although not significant in individual identification, correct genotyping of null repeats is important when using MVR digital codes in parentage analysis. We have therefore characterised these null repeats and show that most null repeats share a common variant repeat sequence. We discuss the possible origins of null repeats and their application to paternity testing and the analysis of minisatellite evolution.