Extranuclear Inheritance

The inheritance of genes in chloroplasts and mitochondria differs from that of nuclear genes in several ways. For instance, organelle genes are characterized by uniparental inheritance. During sexual reproduction, the nuclear genes are inherited from both parents (biparental inheritance). In contrast, the organelle genes are often inherited from only one parent. In animals, this is usually the female parent (maternal inheritance). Mitochondrial genes are inherited maternally because the egg is much larger than the sperm and contains tens of thousands of mtDNA molecules, while the sperm contains only a hundred or so mtDNA molecules. As a result, paternal genes are greatly outnumbered by the maternal genes and can be lost during random replication or other chance events. In addition, in some animals the mitochondrial or mtDNA molecules in the sperm are singled out for degradation in the egg.

Organelle genes are inherited maternally in most plants. In some conifers, mitochondrial genes are inherited maternally, whereas chloroplast genes are inherited paternally. In other conifers, both organelle genomes are inherited paternally. Some other plants (for example, the geranium) and some fungi and protists show a mixed pattern of inheritance. In these organisms, some offspring from a mating inherit organelle genes from only one parent, some only from the other, and some from both parents.

Another way in which extranuclear inheritance differs from nuclear genes is that organelles show vegetative segregation. During the mitotic divisions that produce an adult eukaryote from a single cell, each daughter cell receives one copy of each preexisting nuclear chromosome and gene. The result is that if a cell is heterozygous (has two different versions or alleles of a gene in the nucleus), all of the daughter cells are also heterozygous.

In contrast, different alleles of organelle genes segregate from each other during mitosis. For example, a plant egg with a mixture of normal green and mutant white chloroplasts develops into a plant with a mixture of cells, some of which will contain all green chloroplasts, whereas others will contain all white chloroplasts. This process is called vegetative segregation because it was first discovered in plants, where green and mutant white chloroplasts were observed to segregate during vegetative growth. However, it is now known to occur in all eukaryotes.

Vegetative segregation is the result of two remarkable features of organelle genes. One is random replication. Recall that an organelle contains many copies of its DNA molecule. When mtDNA or cpDNA molecules are replicated before cell division, individual molecules are randomly selected for replication until the total number of molecules has doubled. Consequently, some molecules, and some alleles, are replicated more than others.

The other feature is random partitioning. When an organelle divides, the mtDNA or cpDNA molecules are partitioned (divided up) randomly between the daughter organelles. The result is that heteroplasmic mitochondria or chloroplasts produce homoplasmic daughter organelles with a certain probability in each generation. (An organelle is said to be heteroplasmic if it contains two or more forms of a particular gene. If all the gene copies are identical, it is homoplasmic.) Moreover, when a cell divides, the organelles are partitioned randomly between the two daughter cells, so that a heteroplasmic cell can produce homoplasmic daughters. Over a large number of cell divisions, random replication and random partitioning result in the complete replacement of heteroplasmic cells by homoplasmic cells.