A novel method for forensic DNA investigations: repeat-type sequence analysis of tandemly repeated mtDNA in domestic dogs

A highly variable and heteroplasmic tandem repeat region situated in the mitochondrial mt DNA control region (CR) in domestic dogs and wolves was studied to evaluate its suitability as a forensic genetic marker for analysis of single hairs. The tandem repeat array is composed of three 10-bp repeat types that are distributed so that a secondary DNA sequence is formed. Thus, the region presents two levels of variation: variation in the number of repeats and variation in the secondary DNA sequence of repeat types. Two analysis methods were therefore tested; fragment length analysis and analysis of the sequence of repeat types. Fragment analysis produced unique profiles that could be used to discriminate between blood samples from maternally closely related individuals. However, different hairs from one individual did not have the same fragment profile, and the method is, therefore, not suitable for analysis of single hairs. In contrast, analysis of the repeat type sequences (array types) is highly informative. When different hairs from one individual were studied, identical array types were found. The repeat-type sequence variation was studied among individuals having identical nonrepetitive CR mtDNA sequence variants. Seven, six, and two individuals, representing three different sequence variants, respectively, were analyzed. All these individuals had different array types, which implies a very high genetic variation between individuals in this region. The analysis method considerably improves the exclusion capacity of mtDNA analysis of domestic dogs compared with sequence analysis of non-repetitive DNA.