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Genes in mitochondria and chloroplasts behave quite differently from genes in the nucleus: they are often inherited from only one parent, and they segregate during mitotic cell divisions. Cells contain many copies of each mitochondrial or chloroplast gene, and the replication, recombination, and partitioning of these genes at cell division are much less stringently controlled than is the case for the one or two copies of each nuclear gene. Relaxed control results in random changes in gene frequencies inside single cells or lineages. This may have been the primitive mechanism behind the uniparental inheritance as well as the vegetative segregation of cytoplasmic genes and is still an important factor in many organisms.
C. William Birky (Fri,) studied this question.