RELAXED AND STRINGENT GENOMES - WHY CYTOPLASMIC GENES DONT OBEY MENDELS LAWS

The nuclear genomes of eukaryotes are stringent genomes, defined on the basis of their behavior during the mitotic 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.