2. Extranuclear inheritance of parasites

Extranuclear transmission of viral genomes and symbiotic bacteria is also possible. An example of viral genome transmission is perinatal transmission. This occurs from mother to fetus during the perinatal period, which begins before birth and ends about 1 month after birth. During this time viral material may be passed from mother to child in the bloodstream or breastmilk. This is of particular concern with mothers carrying HIV or Hepatitis C viruses. Examples of cytoplasmic symbiotic bacteria have also been found to be inherited in organisms such as insects and protists.

2. Types of extranuclear inheritance

Three general types of extranuclear inheritance exist. These are vegetative segregation, uniparental inheritance and biparental inheritance.

• Vegetative segregation results from random replication and partitioning of cytoplasmic organelles. It occurs with chloroplasts and mitochondria during mitotic cell divisions and results in daughter cells that contain a random sample of the parent cell’s organelles. An example of vegetative segregation is with mitochondria of asexually replicating yeast cells.

• Uniparental inheritance occurs in extranuclear genes when only one parent contributes organellar DNA to the offspring. A classic example of uniparental gene transmission is the maternal inheritance of human mitochondria. The mother’s mitochondria are transmitted to the offspring at fertilization via the egg. The father’s mitochondrial genes are not transmitted to the offspring via the sperm. Very rare cases which require further investigation have been reported of paternal mitochondrial inheritance in humans, in which the father’s mitochondrial genome is found in offspring. Chloroplast genes can also inherit uniparentally during sexual reproduction. They are historically thought to inherit maternally, but paternal inheritance in many species is increasingly being identified. The mechanisms of uniparental inheritance from species to species differ greatly and are quite complicated. For instance, chloroplasts have been found to exhibit maternal, paternal and biparental modes even within the same species.

• Biparental inheritance occurs in extranuclear genes when both parents contribute organellar DNA to the offspring. It may be less common than uniparental extranuclear inheritance, and usually occurs in a permissible species only a fraction of the time. An example of biparental mitochondrial inheritance is in the yeast, Saccharomyces cerevisiae. When two haploid cells of opposite mating type fuse they can both contribute mitochondria to the resulting diploid offspring.

2. Emerging Discoveries in Cytoplasmic Inheritance

At first glance, the inheritance of nonnuclear genes appears to follow a random pattern of cytoplasmic matter separation. But as we examine this mode of inheritance more closely, new patterns emerge that betray that far more complex processes affect the transfer and maintenance of the organelle genome. It has also become clear that mtDNA may have a close relationship with nuclear genes, and that the integrity of mtDNA may be related to actions coordinated by the cell nucleus. Overall, nonnuclear inheritance is characterized by random patterns of distribution in progeny that appear to follow an entirely different set of rules we are only beginning to understand. In fact, some inheritance patterns are not always due strictly to DNA, nuclear or otherwise. For example, the direction of coiling in snails is determined by a nonuniform distribution of cytoplasmic factors in the early embryo. Early cell divisions result in irregular distribution of these factors in the embryonic cells, which has been linked to the inheritance of left- or right-handed coiling.

Together, all cellular sources of DNA and the intracellular factors that are inherited from parent to offspring interact to influence the heredity of traits. The complexity of all these parts working together makes inheritance relevant even today. Mendel principles helped guide the way for understanding the basic inheritance of alleles, but the complexity of how genetic, epigenetic, and environmental factors intertwine to control distinct phenotypes continues to be explored by scientists every day.