While nuclear chromosome genes play a significant and central role in the inheritance of most traits across generations, they are not the sole agents responsible for inheritance.
Experimental evidence has indicated the existence of extranuclear genes or DNA molecules located in the cytoplasm of many prokaryotic and eukaryotic cells.
In bacterial cells like Escherichia coli (E. coli), there is a single primary chromosome located in the nucleoid region, but additional DNA elements known as plasmids are often present in the cytoplasm.
Eukaryotic cells contain their primary set of chromosomes within the nucleus, but also possess additional DNA molecules or chromosome-like structures within organelles such as mitochondria and chloroplasts.
Extranuclear inheritance, also called cytoplasmic inheritance, refers to the transmission of genetic information that occurs outside the boundaries of the cell nucleus.
This type of inheritance represents a form of non-Mendelian inheritance, where traits are passed from parent to offspring through cytoplasmic components rather than through chromosomal DNA found in the nucleus.
Extranuclear inheritance is commonly known by several terms, including non-Mendelian inheritance, non-chromosomal inheritance, uniparental inheritance, maternal inheritance, extra-chromosomal inheritance, cytoplasmic inheritance, and extra-nuclear inheritance.
Cytoplasmic Factors of Extranuclear Inheritance
Cytoplasmic factors involved in extranuclear inheritance are genetic elements found in the cytoplasm rather than in the nucleus.
These factors are typically independent and self-replicating nucleic acids, distinct from chromosomal (nuclear) genes based on their cellular location.
Unlike nuclear genes, cytoplasmic factors have their own unique nucleotide sequences and replication systems.
Major examples of cytoplasmic factors include:
Mitochondria
Chloroplasts
Plasmids
Mitochondria
Mitochondrial genomes vary in size among different organisms.
In animals, the mitochondrial genome typically ranges from 13 to 18 kilobases (kb) in size.
Fungal mitochondrial genomes are generally around 75 kb.
Mitochondrial genomes in higher plants are significantly larger, ranging from approximately 300 to 500 kb.
Each mitochondrion contains multiple copies of its genome, usually between 5 to 20 copies of the mitochondrial chromosomes.
The number of mitochondria varies across different human cell types, depending on the energy requirements of the cells.
Human mitochondrial DNA (mtDNA) encodes a total of 37 genes.
Chloroplast
Chloroplast genomes typically range in size from 130 to 150 kilobases (kb).
Chloroplasts contain more genes compared to mitochondria, with approximately 110 genes versus 37 in mitochondria.
The majority of chloroplast genes are associated with the process of photosynthesis.
In corn (Zea mays), each cell contains between 20 to 40 chloroplasts.
Each chloroplast in corn has around 20 to 40 chromosomes.
Chloroplast DNA can constitute up to 15% of the total cellular DNA.
Types of Extranuclear Inheritance
1. Maternal (organelle) Inheritance
Maternal (organelle) inheritance involves DNA located in mitochondria or chloroplasts, which determines the phenotype of the offspring.
These phenotypes emerge because the organelles are inherited exclusively from the egg, meaning there is only a maternal influence on the resulting traits.
Mitochondria and chloroplasts are passed along with the egg’s cytoplasm from the maternal parent.
Examples of maternal (organelle) inheritance include:
Shell coiling in snails.
Variegation in the leaves of the 4 o’clock plant (Mirabilis jalapa).
Eye pigmentation in water fleas and flour moths.
Maternal effects occur when the genotype or phenotype of the mother directly influences the phenotype of her progeny, regardless of the offspring's genotype.
For example, older salmon produce larger eggs that contain more nutrients, leading to larger fry at the time of hatching.
2. Extra-Nuclear Inheritance by Endosymbionts
Extranuclear inheritance by endosymbionts refers to certain intracellular parasites, like bacteria or viruses, that live in a symbiotic relationship with host cells.
These endosymbionts are self-replicating and often appear as cytoplasmic inclusions.
Sometimes, they are transmitted in an infection-like manner while maintaining their own hereditary continuity.
The symbiotic or parasitic association with the host results in the transfer of certain phenotypes to the offspring through cytoplasmic inheritance.
Examples of endosymbiont-based inheritance include:
Sigma virus in Drosophila.
Kappa particles in Paramecium.
3. Uniparental inheritance
Uniparental inheritance is the transmission of hereditary traits from only one parent.
It occurs in extranuclear genes when only one parent contributes organelle DNA to the offspring.
Types of uniparental inheritance include:
Maternal inheritance: Traits are passed solely through the mother.
Paternal inheritance: Traits are passed solely through the father.
In human fertilization, mitochondrial DNA is maternally inherited through the egg, and the sperm’s mitochondrial genes are not transmitted to the offspring.
Chloroplast DNA may also be inherited uniparentally during sexual reproduction.
The mechanisms of uniparental inheritance vary greatly among species and can be complex.
In some species, chloroplasts may follow maternal, paternal, or even biparental inheritance patterns—even within the same species.
4. Biparental Inheritance
Biparental inheritance of extranuclear genes occurs when both parents contribute organelle DNA to the offspring.
This type of inheritance is less common than uniparental inheritance and usually occurs only occasionally in species where it is permitted.
An example of biparental mitochondrial inheritance is seen in yeast, where two haploid cells of opposite mating types fuse and both contribute mitochondria to the resulting diploid cell.
References
Verma, P. S., & Agrawal, V. K. (2006). Cell Biology, Genetics, Molecular Biology, Evolution and Ecology (1st ed.). New Delhi: S. Chand & Company Ltd.
Fatchiyah. (n.d.). Lecture 3 – Extranuclear Inheritance. Retrieved from [University of Brawijaya Lecture Notes]: http://fatchiyah.lecture.ub.ac.id/files/2011/04/Lect-3-ok-Extranuclear-Inheritance.pdf
Abbas, R. (n.d.). Extra-Nuclear Inheritance [Slide presentation]. Retrieved from SlideShare: https://www.slideshare.net/RIZWANABBAS3/extra-nuclear-inheritance
Emigh, M. (n.d.). Extranuclear Inheritance [Course material, GN411]. North Carolina State University. Retrieved from: https://projects.ncsu.edu/cals/course/gn411/emigh411/extranuclear.pdf
Keerthana, N. (n.d.). Extranuclear Inheritance [Slide presentation]. Retrieved from SlideShare: https://www.slideshare.net/keerthanan77/extranuclear-inheritance
