In mammals, each somatic diploid cell has two copies of the nDNA genomes, with one inherited from each parent. On the other hand, the progeny inherits its mtDNA directly from the mother (Giles et al., 1980). Mechanisms behind the maternal inheritance of mtDNA include the reduction of paternal (spermatic) mtDNA during spermatogenesis, the simple dilution of spermatic mtDNA at fertilization (an overwhelming copy number of oocyte mtDNA relative to spermatic mtDNA) and ubiquitin-mediated proteolysis of spermatic mitochondria and the active digestion of spermatic mtDNA within a fertilized ovum. (Kaneda et al., 1995; Hershko and Ciechanover, 1998; Nishimura et al., 2006). Due to these numerous safeguards, the paternal mtDNA that enters an oocyte becomes undetectable after the fertilized egg undergoes its first mitotic division.
Whether mammalian mtDNA undergoes recombination or not is a longstanding question. In recent experiments using human somatic hybrid cells and mice carrying two different mtDNA, only three of 318 clones of mtDNA purified from mouse tissues corresponded to recombinant mtDNA, whereas no recombinants were found in human somatic hybrid cells. These results strongly suggest that recombination can occur within mammalian cells but at a very low frequency or at an operationally undetectable level. This implies that recombinant mtDNA observed in mice might be gene conversion products resulting from the repair of damaged mtDNA molecules (Sato et al., 2005)
Maternal inheritance and a very low recombination rate means that mtDNA genomes are essentially clonal copies of the mother's mtDNA genomes (provided that the sequence of all mtDNA molecules within an oocyte are the same, the recombinant mtDNA structure will also be the same). From a forensic viewpoint, maternal inheritance can be a helpful tool in the identification of a body or the remains of a missing person. The missing person's biological mother, siblings and maternal relatives all have the same mtDNA sequence, with few exceptions resulting from heteroplasmy as described later. Therefore, biological samples (e.g. blood, buccal swabs) taken from these individuals can provide reference samples for the identification of the missing individual. On the other hand, since a progeny's mtDNA contains no paternal information, mtDNA typing cannot be used for paternity tests.
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