https://phys.org/news/2019-01-differences-genes-geographic-mitochondrial-function.html

The researchers focused specifically on genes used by mitochondria. To perform their functions, mitochondria rely on genes encoded in the DNA of the mitochondria itself and on genes that are encoded within the nucleus of the cell. Because mitochondrial and nuclear DNA are inherited in different ways—nuclear DNA is roughly equally passed on from mother and father, but mitochondrial DNA is exclusively maternal—there are greater opportunities for differences in origin of mitochondria-related genes in the nuclear genome compared to genes in the mitochondrial DNA itself.

The researchers first investigated whether discordance—the degree of difference in geographic origin between the mitochondrial and nuclear genomes—can affect a trait. They found that increasing amounts of discordance were related to decreasing numbers of copies of mitochondrial DNA in a cell, which could imply reduced efficiency of mitochondrial DNA replication.