Extranuclear Inheritance

Extranuclear Inheritance

In the nineteenth century, biologist Gregor Mendel established patterns of heredity in sexually reproducing organisms on the basis of equal genetic contributions from both parents. It is now known, however, that there are exceptions to Mendel's laws, and that DNA other than that contained in the nucleus of the gametes can influence the characteristics of the offspring.

Both the chloroplasts of photosynthetic organisms and the mitochondria of all eukaryotic organisms contain their own genomes. The small number of genes in these organelles code for tRNAs, rRNAs, and proteins used in photosynthesis and oxidative phosphorylation, respectively. These genes are inherited independent of the nuclear genes, and are contributed almost exclusively by the maternal gamete, or egg. One of the earliest observations of this maternal inheritance in plants was in the four o'clock flower, Mirabilis jalapa, in which the leaf color depends solely on that of the plant that contributes the ovule. Maternal inheritance has also been observed in fungi, protists, and animals. A hallmark of maternal inheritance is that if there is a mutation that changes the phenotype, all offspring of affected mothers, but no offspring of affected fathers, show the changed phenotype. In other words, males can inherit a mutation from their mother, but cannot pass it along. Until recently, scientists believed that the mitochondria in sperm did not enter the egg upon fertilization. Recent evidence contradicts this long-held view. It now appears that the sperms' mitochondria, which are certainly necessary for the journey to the egg, are tagged with the molecule ubiquitin while they are still in the male's reproductive tract. Organelles bearing a ubiquitin label are broken down and recycled by cells. Soon after fertilization, before the third embryonic cleavage, paternal mitochondria are destroyed. Thus, only maternal mitochondria contribute to the phenotype of the offspring.

In the process of embryogenesis, cell division occurs rapidly and often. The many mitochondria in each cell are randomly allocated to the two cells resulting from each division. In normal humans, virtually all of mitochondrial DNA (mtDNA) is identical, a condition called homoplasmy. If a mutation occurs in the mtDNA, however, it may be transmitted by chance to some cells but not others during the division of the cytoplasm. Furthermore, different cells in the same animal may have different proportions of normal and mutated mtDNA. The presence of two populations of mtDNA, normal and mutated, in one individual, is called heteroplasmy. Mutations in the mtDNA may accumulate with age, causing an inability to produce sufficient ATP, which in turn is likely responsible for many aspects of aging. There are a number of human diseases caused by mutations in the mtDNA. Because the mitochondria are responsible for ATP production, tissues with a high energy requirement, including muscle and nervous tissue, show the greatest effects from such mutations. One disease caused by mutations in mtDNA and passed only from mother to child is Leber's Hereditary Optic Neuropathy, which causes sudden bilateral blindness. Another is Kearns-Sayre Syndrome, which causes loss of vision and hearing, and heart conditions. Still another disease caused by mutations in mtDNA is MERFF, which stands for myoclonic epilepsy and ragged red fiber, in which the muscle fibers have a characteristic appearance upon microscopic examination.

In another type of extranuclear inheritance, called infectious heredity, a symbiotic or parasitic organism lives in the cytoplasm of a cell, and its DNA is transferred to progeny cells along with the cytoplasm. An example of this process can be seen in Drosophilae, or fruit flies. Certain flies are sensitized to and unable to recover from CO₂ anesthesia, as normal flies do. Instead, these sensitive flies become paralyzed and die. All offspring of sensitive mothers are sensitive, as well. Extracts of the sensitive flies can be injected into normal flies, making them sensitive. The agent that causes this effect is a virus called sigma, which is passed from mother to offspring. A similar phenomenon has been observed in the protist Paramecium aurelia, which houses kappa particles. These particles contain DNA that encodes the toxin paramecin, which is tolerated by its host cell, but toxic to other cells.