粒線體mtDNA包含數百套獨立之mtDNA, 以不均勻方式分配到卵細胞中
不同卵細胞之mtDNA可能具有極大之差異性

Mitochondrial Transmission

The prokaryotic origins of the mitochondria also influence their genetics. In animals, most cells contain many copies of mtDNA, and the transmission can be better imagined as thousands of independent genomes being segregated by stochastic mechanisms and relaxed, uncoupled replication, which is in stark contrast to the structured meiosis and mitosis of eukaryotic nuclear genomes [28]. Despite the high copy number of mtDNA, it was noted very early that mtDNA of vertebrates can switch from one type to another in a very short period of time, even in a single generation [29]–[31]. To explain this very rapid shift, the concept of the mitochondrial bottleneck was developed (Figure 1). In the development of the germline, a large population of mtDNAs are subsampled to a relatively small number. This subselected population may differ markedly from the source population, allowing rare alleles to, by chance, suddenly come to dominate the mtDNA pool. With a sufficiently small sample, as observed in Holstein cows, single-generation shifts of mtDNA genotype are possible [30], [31]. Such a bottleneck phenomenon has since been detected widely in vertebrates [32]–[34] and even in Drosophila [35]–[37], though the size of this subsampling bottleneck appears to be an order of magnitude larger in flies than in vertebrates.

http://www.ncbi.nlm.nih.gov/pmc/articles/PMC4191934/


Schematic representation of the mitochondrial bottleneck.

A large number of mtDNA molecules are present in the maternal mtDNA pool (bottle). The figure depicts two genotypes (blue and yellow circles). The generation of an oocyte involves subsampling of just a few mtDNA molecules from the maternal mtDNA pool (buckets on conveyor belt), which will be transferred to the developing oocyte and extensively replicated. The effect of this poorly understood bottleneck mechanism is that the proportion of the two genotypes can vary widely between oocytes.