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The segregation of mitochondrial genomes and the inheritance of mitochondrial DNA are constant matters of debate. To obtain more information about this issue and to answer the question whether or not it is possible to distinguish mitochondrial DNA (mtDNA) samples from monozygous individuals by analysing heteroplasmic length variants, 290 monozygous and 121 dizygous twin pairs and 34 sets of multiples were studied by RFLP and partly by direct sequencing. A factor D describing the respective pattern of length variants in a given sample was also calculated. The results show that monozygous individuals exhibit a significantly lower median and closer distribution of D than non-monozygous siblings. Thus, a differentiation of mtDNA samples from monozygous twins by this trait is not possible. The high percentage of heteroplasmic individuals, the low median of the D values and the unexpectedly very similar distribution of length variants in monozygotic individuals support the existence of a relatively wide bottleneck or the assumption of a regeneration of length heteroplasmy following a tight bottleneck and agree with a random segregation of mtDNA genomes in dividing oocytes.

The nature of mitochondrial DNA heteroplasmy is still unclear. It could either be caused by two mitochondrial DNA (mtDNA) haplotypes coexisting within a single cell or by an admixture of homoplasmic cells, each of which contains only one type of mtDNA molecule. To address this question, single lymphocytes were separated by flow cytometry assisted cell sorting and analyzed by cycle sequencing or minisequencing. To attain the required PCR sensitivity, the reactions were carried out on the surface of chemically structured glass slides in a reaction volume of 1-2 microl. In this study, blood samples from two healthy donors showing mitochondrial point heteroplasmy in direct sequencing (195Y and 234R, respectively) were analyzed. Nearly 96% of single lymphocytes tested were found to be in a homoplasmic state, but heteroplasmic cells were also detected. These results suggest that mitochondrial point heteroplasmy in blood may well be mainly due to the mixture of homoplasmic cells.

The canine mitochondrial DNA (mtDNA) control region of 133 dogs living in the area around Innsbruck, Austria was sequenced. A total of 40 polymorphic sites were observed in the first hypervariable segment and 15 in the second, which resulted in the differentiation of 40 distinct haplotypes. We observed five nucleotide positions that were highly polymorphic within different haplogroups, and they represent good candidates for mtDNA screening. We found five point heteroplasmic positions; all located in HVS-I and a polythymine region in HVS-II, the latter often being associated with length heteroplasmy. In contrast to human mtDNA, the canine control region contains a hypervariable 10 nucleotide repeat region, which is located between the two hypervariable regions. In our population sample, we observed eight different repeat types, which we characterized by direct sequencing and fragment length analysis. The discrimination power of the canine mtDNA control region was 0.93, not taking the polymorphic repeat region into consideration.

The highly decomposed remains of a corpse were identified by the amplification and direct sequencing of mitochondrial (mt) DNA. Degraded DNA was extracted from bone fragments and a necrotic skin sample and amplified at 2 hypervariable segments within the mitochondrial non-coding region using 2 rounds of nested PCR. Both strands of the amplified regions were sequenced and compared with each other to ensure fidelity of the data. Sequences from the bone and skin were found to be identical and matched data generated from a blood sample provided by a presumptive sister of the deceased. Thus, this evidence was consistent with the sister and the deceased being related.