The elimination of sperm mtDNA also ensures that one population of mtDNA is inherited. Previously, it was thought that all copies within an individual were identical and that very few copies would harbour variants (54). However, it is becoming increasingly clearer that, across mammalian species, there are a series of low level variants that are either inherited and passed down through several generations or are de novo (55-57). In either case, they do not necessarily affect the offspring’s phenotype and appear to persist across generations (55-57). If the variant load increases then compensatory mechanisms are often recruited to offset this outcome, such as increases in mtDNA copy number that make more wild type molecules available for transcription (55). Nevertheless, the inheritance of one population of mtDNA (namely the presence of wild type molecules and those molecules that contribute to low level variants) defers from ‘genetic conflicts’ that would arise when two different mitochondrial genomes co-exist within a cell or tissue (21, 40, 58). These different mitochondrial genomes have the potential to encode very different amino acids that would affect electron transfer chain assembly and the generation of ATP through OXPHOS. They would also affect other mitochondrial functions such as steroidogenesis, calcium storage and regulation of apoptosis as failure by the electron transfer chain to establish the mitochondrial membrane potential would result in loss of mitochondrial function that would, in turn, affect cellular function. Consequently, the uniparental inheritance of the mitochondrial genome, namely through the female germ line, ensures that only one population of mtDNA is transmitted to the offspring to promote effective production of cellular energy and appropriate mitochondrial and cellular function.

The diversity of mitochondrial genomes that exist within a species is reflected by their classification into different mtDNA haplotypes (59). In humans, the diversity most likely arose as our ancestors migrated out of a central location, namely a region in Africa, to various different parts of the world (60) (Figure 8). As a result, their mitochondrial genomes, which reflect our mitochondrial genomes today, underwent mutation to adapt to the environments that we inhabit. Indeed, mtDNA haplotypes can confer both advantages and disadvantages to individuals. For instance, some mtDNA haplotypes confer adaptation to warmer climates, while others confer adaptation to colder climates (61). Likewise, some mtDNA haplotypes are protective or predispose individuals to diseases such as cardiomyopathy (62, 63), cancer (64), diabetes (65) and associated complications, such as retinopathy, neuropathy, nephropathy, and renal failure (66); and the onset of AIDS (67), Parkinson’s (68, 69) and Alzheimer’s (70, 71). However, as migration continues and we are required to adapt to our new environments, it would be interesting to know if our mtDNA haplotypes further evolve over subsequent generations.