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 definition

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

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).

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.

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.

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.

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.

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.