The total genetic complement of our species is called “human genome”. It comprises a set of different DNA molecules, corresponding to 25 different types of chromosomes (22 autosomes, X chromosome, Y chromosome, mitochondrial DNA) which are present within our cells. All these DNA molecules are transmitted from cell to cell and from generation to generation: chromosomes by mitosis and meiosis, mitochondrial DNA by replication followed by mitochondrial fission and by random inclusion in the cytoplasm of daughter cells after cell cleavage. Since mitochondria of the zygote are almost exclusively contributed by the cytoplasm of the female gamete, genes carried by mitochondrial DNA (mtDNA) follow the rules of “maternal inheritance” or “cytoplasmic inheritance”. In principle, mutations in mitochondrial DNA will be transmitted to all the children of the same mother, but only daughters will pass a mtDNA mutation to next generation. On the contrary, mutations in DNA carried by chromosomes will follow the rules dictated by chromosomal duplication, recombination and segregation.