A Basis for Anomalous Band Patterns Encountered During DNA STR Profiling

Abstract

Since 1995 the Forensic Science Service (FSS) has carried out DNA profiling of reference samples for the UK National DNA Database (1) and in forensic casework using two multiplex STR profiling systems (2,3). During this period, profiles with anomalous banding patterns, although comparatively rare, have been encountered regularly. The FSS has collected instances of triallelic patterns and aberrant diallelic patterns. A systematic examination of these patterns has provided insight into their underlying genetic cause. The triallelic patterns could be classified into two types based on the relative intensities of their component alleles. In the Type 1 pattern the alleles were of uneven intensity, whereas in the Type 2 pattern, all three alleles were of even intensity. Evidence is presented that the more frequent Type 1 pattern is the result of somatic mutation at a heterozygous locus, and the Type 2 pattern is the result of a localized chromosomal rearrangement at a heterozygous locus. Directly from the Type 1 pattern, it was possible to deduce the size difference between the progenitor and mutated allele. All mutational changes were found to be multiples of four nucleotides, suggesting the loss or addition of one or more tetrameric repeat units. Aberrant diallelic patterns were identified by analysts due to an unexpectedly large difference in intensity between alleles at a heterozygous locus. While some of these diallelic patterns are likely caused by the same genetic phenomena described above occurring at a homozygous locus, others are demonstrated to be caused by a mutation in the primer binding sequence, leading to a reduction in amplification efficiency of one allele. It is concluded that based on a visual inspection of a profile, it is possible to infer a likely genetic basis directly from the triallelic pattern. By contrast, the aberrant diallelic patterns can be due to any one of a number of possible genetic effects.

Genotyping of Vitis vinifera L. within the Slovak national collection of genetic resources

“…An unusual triallelic pattern was also detected within analysed accessions -at the locus ISV2 in the cultivar Irsai Oliver (Figure 2), representing Type 2 of triallelic pattern [20]. Such somatic mutation and its accumulation were designated as a way of diversification process and phylogeny [21] and as a slow but constant evolution ongoing in grapevine [2].…”

A Large Population Genetic Study of 15 Autosomal Short Tandem Repeat Loci for Establishment of Korean DNA Profile Database

“…These unusual genotypes were confirmed by repeated analysis using a different multiplex PCR system (Powerplex 16 system, Promega, USA). The triallelic patterns are usually classified into two types based on the relative intensities of their component alleles: type 1 with an uneven height of the peaks and type 2 with an even height of the peaks (Clayton et al, 2004). In this study, three variants were type 2 (even) and one variant was type 1 (uneven).…”

“…Non-standard DNA profiles can result from biological events such as allele dropout, slippage, conversion and copy number variations (Freeman et al, 2006). Thus far, a few autosomal STR data pertaining to variant alleles have been reported in forensic communities (Clayton et al, 2004;Dauber et al, 2009;Egyed et al, 2000).…”

One person with two DNA profiles: a(nother) case of mosaicism or chimerism

“…Natural chimerisms resulting from the fusion of zygotes [7], from exchanges of blood between fetuses in utero, e.g., [8][9][10], or from double parental contribution, e.g., [11][12][13][14] as well as mosaicism are compatible with our data. Mosaics originate from somatic mutation and have generally only one STR locus altered [15,16], but cancer may considerably increase the mutability of genetic markers, e.g., [17][18][19][20][21]. For instance, STR markers were altered respectively in 33% and 68% of oral and gastrointestinal cancer patients, with more than half of the patients having several loci altered [18,21].…”