Relaxed and Stringent Genomes: Why Cytoplasmic Genes Don't Obey Mendel's Laws

The nuclear genomes of eukaryotes are stringent genomes, defined on the basis of their behavior during the mltotic cell cycle: each chromosome Is copied exactly once in the S phase, and each daughter cell receives one copy at the ensuing mitotic division. In contrast, a number of lines of genetic and physical evidence show that mitochondria and chloroplasts have relaxed genomes, defined as those in which DNA molecules (each with a complete copy of the organelle genome) are chosen randomly for replication, and partitioned randomly to daughter cells at cell division. Many features of the non-Mendelian Inheritance of organelle genes are consequences of relaxed replication and partitioning, which explain why alleles of organelle genes segregate during mitotic as well as meiotic divisions; why organelle genes can be subject to selection within and among cells as well as among Individuals; and why organelle genes tend to be inherited uniparentally even when both parents contribute organelle genomes to the zygote.