Non Nuclear Inheritance

There is a good chance that you have heard a great deal about how humans inherit genetic material from their nucleus at this point in time. However, how is the genetic material contained in mitochondria and chloroplasts (in plants) inherited? Non-nuclear inheritance refers to the inheritance of mitochondrial and chloroplast DNA.

Non Nuclear Inheritance Non Nuclear Inheritance

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Table of contents

    Non-nuclear inheritance definition

    First, let's look at the definition of non-nuclear inheritance.

    Non-nuclear inheritance is the inheritance of traits from DNA located outside the nucleus, in the mitochondria (mtDNA) and chloroplasts (cpDNA) of the cell.

    Non-nuclear inheritance is a type of maternal inheritance because, when it comes to cytoplasmic genes (mtDNA and cpDNA), only the female gamete (mother) contributes the organelles along with the cytoplasm. In other words, none of the organelle DNA in the zygote comes from the male parent.

    • Remember that only plant cells contain chloroplasts and chloroplast DNA (cpDNA).

    Uniparental inheritance is a special mode of inheritance in which progeny inherits organelle genes exclusively from one parent but not the other.

    Non-nuclear inheritance has many names such as extranuclear inheritance, organelle inheritance, maternal inheritance, and cytoplasmic inheritance. It is also a type of non-Mendelian inheritance.

    There are three main characteristics of non-nuclear inheritance:

    1. Non-nuclear inheritance does not show typical Mendelian ratios (no meiotic segregation involved) because the offspring generally receives cytoplasmic genes from one parent only.
    2. Mitochondrial genes show maternal inheritance.
    3. Lack of chromosomal location (mitochondrial and chloroplast genes cannot be mapped to the chromosomes in the nucleus).

    Non-nuclear inheritance genetics

    Let's talk about the non-nuclear inheritance of mitochondrial DNA (mt-DNA). We can also call it mitochondrial inheritance. In animals, all mitochondria are transmitted by the egg, whereas in plants, they are transmitted by the ovule (maternal inheritance). This maternal inheritance of mitochondrial DNA can be seen in Figure 1.

    Mitochondria are organelles responsible for cellular respiration and production of adenosine triphosphate (ATP). Mitochondria contain its own DNA (mt-DNA) and ribosomes.

    Although very uncommon, sometimes mitochondrial inheritance can be biparental. Biparental inheritance happens when both parents contribute organellar DNA to the offspring. For example, in the yeast Saccharomyces cerevisiae, the inheritance of mtDNA is a biparental because when the two haploid cells of the fungus fuse, the mtDNAs mix, and both mating types contribute mtDNA to the diploid offspring (Figure 2).

    Non-nuclear inheritance is also seen in chloroplasts. The DNA in the chloroplast is known as chloroplast DNA (cp-DNA) or plastid DNA (pt-DNA). Chloroplast DNA is also maternally inherited.

    A chloroplast is an organelle found in plants that absorbs sunlight and makes use of it to synthesize sugars (glucose) from carbon dioxide and water.

    Let's take a look at the chloroplast inheritance in Mirabilis jalapa (four o'clock) plants. Sometimes a mutation in the chloroplast genes (cp-DNA) of this plant causes it to have white leaves/branches instead of the normal green color we are used to seeing in plants. However, some plants are variegated, meaning that they contain both green and white patches in the same plant.

    In these variegated plants, there were three types of female (egg) cells being produced:

    • Normal eggs (green chloroplasts)
    • Mutant eggs (white/colorless chloroplasts)
    • Mixed eggs (green and white chloroplasts)

    So, assuming that the pollen cell of males contributes no information, the zygote will inherit the mother's cp-DNA (Figure 3)!

    Endosymbiotic Theory

    As we discussed earlier, mitochondria and chloroplasts have their own DNA and ribosomes. But, why are mtDNA and cpDNA the only types of DNA in the cytoplasm? In accordance with the endosymbiotic theory, mitochondria and chloroplasts were once independent living bacteria!

    Scientists believe that around 1.5 billion years ago, an aerobic bacterium (a bacterium that uses oxygen) was engulfed by an anaerobic host cell, making a symbiotic relationship. As time passed, the aerobic bacterium lost many genes/abilities, eventually developing into what we call mitochondria.

    A symbiotic relationship is a relationship in which organisms from two separate species depend on each other for survival.

    Similarly, a photosynthetic cyanobacterium is believed to have been engulfed by a host cell, and over time, evolved to become a chloroplast.

    The evidence supporting this endosymbiotic theory is that, as in prokaryotes, both mitochondria and chloroplasts have small, circular DNA, 70S ribosomes and replicate via binary fission. They also have inner and outer membranes consistent with engulfment.

    Nuclear inheritance vs. cytoplasmic inheritance

    Nuclear inheritance is simply the inheritance of the chromosomes found in the nucleus of the cell. Nuclear inheritance does follow Mendelian rules of inheritance and involves genes found on the chromosomal DNA.

    Nuclear inheritance is the inheritance or transmission of nuclear genes from parent to offspring.

    Contrary to what we saw in non-nuclear inheritance (also known as cytoplasmic inheritance), nuclear inheritance is actually mediated by both maternal and paternal inheritance. In other words, the mother's nucleus and the father's nucleus equally contribute to nuclear inheritance.

    Figure 4 shows a simple diagram of the inheritance of cytoplasmic and nuclear genes. As expected, the diploid zygote inherits the mtDNA and cpDNA (in the case of plants) from the female gamete, and a nucleus containing one nuclear allele of each gene from each parent as a result of fertilization.

    Non-nuclear inheritance example

    An interesting example of non-nuclear inheritance can be seen in Neurospora crassa, an orange bread mold. In this fungus, a mutant strain called poky has a slow-growth phenotype, whereas a wild-type strain has normal growth.

    Researchers found that by crossing a poky female and a normal male, all the progeny would be poky. In the reciprocal cross (normal females x poky male), all progeny would be wild-type. This result shows that the inheritance of slow or normal growth was clearly maternal. Moreover, gene sequencing found that the poky phenotype was attributed to a mutation of a ribosomal RNA gene in mitochondrial DNA (Figure 5).

    Another common example of non-nuclear inheritance is seen in a genetic mutation that impairs oxidative phosphorylation, reduces ATP output and results in nerve deterioration. Mothers possessing defects in the mitochondria pass the defect to their offspring.

    Non-nuclear inheritance diseases

    Now that we know what non-nuclear inheritance is, let's explore some diseases associated with mutations in the mitochondrial genes.

    Let's start with Leber hereditary optic neuropathy. This disorder is characterized by a loss of central vision, eventually leading to blindness. Figure 6 shows a pedigree diagram of the inheritance of Leber hereditary optic neuropathy. Notice that an affected mother (colored circles) passes the mutated mt-DNA to all her sons and daughters, but only the daughters are able to transmit the mutated DNA (and thus, the disorder) further to their progeny.

    Kearns-Sayre syndrome is another example of a mitochondrial disorder caused by defective mt-DNA (large deletions of mitochondrial DNA), and it results in paralysis of eye muscles and degeneration of the retina.

    Another disorder associated with mitochondrial inheritance is myoclonic epilepsy with ragged-red fibers (MERRF). MERRF is a rare disorder in which 80% of patients suffering from it have a mutation of mitochondrial DNA. It affects mostly the muscles and the nervous system, causing twitches, weakness and progressive stiffness.

    Non-Nuclear Inheritance - Key takeaways

    • Non-nuclear inheritance is the inheritance of traits from DNA located outside the nucleus, in the mitochondria (mtDNA) and chloroplasts (cpDNA) of the cell.
    • Non-nuclear inheritance is a type of maternal inheritance because only the female gamete (mother) contributes the organelles along with the cytoplasm.
    • Nuclear inheritance differs from non nuclear inheritance in that nuclear inheritance is mediated by both maternal and paternal inheritance. In other words, the mother's nucleus and the father's nucleus equally contribute to nuclear inheritance.


    References

    1. AP Biology - AP Central | College Board. (2017, May 30). AP Central. https://apcentral.collegeboard.org/courses/ap-biology
    2. Berger, K. H., & Yaffe, M. P. (2000). Mitochondrial DNA inheritance in Saccharomyces cerevisiae. Trends in Microbiology, 8(11), 508–513. https://doi.org/10.1016/s0966-842x(00)01862-x
    3. Pack, P. E. (2017). CliffsNotes AP biology. Houghton Mifflin Harcourt.
    4. Urry, L. A., Cain, M. L., Wasserman, S. A., Minorsky, P. V., Orr, R. B., & Campbell, N. A. (2020). Campbell biology (12th ed.). Pearson.
    5. Russell. (2009). IGenetics, 3rd ed. Pearson.
    6. Cotran, R. S., Kumar, V., & Robbins, S. L. (2015). Robbins and Cotran Pathologic Basis of Disease (9th ed.). Saunders Elsevier.
    Frequently Asked Questions about Non Nuclear Inheritance

    Where is the DNA found in non nuclear inheritance? 

    In non nuclear inheritance, DNA is found in the mitochondria (of both animals and plants) and in the chloroplast (in plant cells).

    From which parent is non nuclear DNA inherited?

    Non nuclear DNA is inherited by the female parent.

    What is non nuclear inheritance?

    Non-nuclear inheritance is the inheritance of traits from DNA located outside the nucleus, in the mitochondria (mtDNA) and chloroplasts (cpDNA) of the cell. 

    Where are non-nuclear genes found?

    Non-nuclear genes are found in the mitochondrial and chloroplasts.

    Non nuclear inheritance answers 

    Non-nuclear inheritance answers the question of how mtDNA and cpDNA are inherited. 

    Test your knowledge with multiple choice flashcards

    True or false: Nuclear inheritance is the inheritance of both nuclear and non nuclear DNA.

    ______ inheritance is the inheritance of traits from DNA located outside the nucleus, in the mitochondria (mtDNA) and chloroplasts (cpDNA) of the cell.

    Non-nuclear inheritance is a type of ______  inheritance.

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