Mitochondrial and chloroplast DNA and why its inheritance does not follow Mendelian patterns.

Introduction

If you were asked to name the organelle that contains DNA, what would you say? If you said the nucleus, you'd definitely get full points, but the nucleus is not the only source of DNA in most cells.
Instead, DNA is also found in the mitochondria present in most plant and animals cells, as well as in the chloroplasts of plant cells. Here, we'll explore how mitochondrial and chloroplast DNA are inherited.

Mitochondrial and chloroplast DNA

The DNA molecules found in mitochondria and chloroplasts are small and circular, much like the DNA of a typical bacterium. There are usually many copies of DNA in a single mitochondrion or chloroplasts.
Diagram of a eukaryotic cell, showing the location of the mitochondria (floating in the cytosol, outside of and separate from the nucleus), and zooming in on the mitochondria to show the circular DNA molecules inside them.
Image modified from "Mitochondrial DNA," NIH Talking Glossary of Genetic Terms, NHGRI (public domain).
Similarities between the DNA of mitochondria and chloroplasts and the DNA of bacteria are an important line of evidence supporting the endosymbiont theory, which suggests that mitochondria and chloroplasts originated as free-living prokaryotic cells.

How is non-nuclear DNA inherited?

Here are some ways that mitochondrial and chloroplast DNA differ from the DNA found in the nucleus:
  • High copy number. A mitochondrion or chloroplast has multiple copies of its DNA, and a typical cell has many mitochondria (and, in the case of a plant cell, chloroplasts). As a result, cells usually have many copies – often thousands – of mitochondrial and chloroplast DNA.
    Not necessarily. There’s good evidence that individual mitochondria can fuse and exchange genetic material with one another, meaning that not all of the DNA copies in a mitochondrion are necessarily identical1^1. In plants, chloroplast fusion appears less common, so individual chloroplasts may tend to be genetically homogeneous (though the various chloroplasts in a cell may still differ from one another)2^2. Of course, it’s also possible for the DNA copies in a genetically homogeneous organelle to become non-identical if a random mutation occurs in one of them.
  • Random segregation. Mitochondria and chloroplasts (and the genes they carry) are randomly distributed to daughter cells during mitosis and meiosis. When the cell divides, the organelles that happen to be on opposite sides of the cleavage furrow or cell plate will end up in different daughter cells3^3.
  • Single-parent inheritance. Non-nuclear DNA is often inherited uniparentally, meaning that offspring get DNA only from the male or the female parent, not both4^4. In humans, for example, children get mitochondrial DNA from their mother (but not their father).

Chloroplast inheritance: Early experiments

At the turn of the 20th century, Carl Correns, a German botanist, did a series of genetic experiments using four o’clock plants (Mirabilis jalapa). We now know that his work demonstrated how chloroplast DNA is passed on from cell to cell and from parent to offspring—though Correns himself didn't know it at the time5^5!

Correns' experiments

The Mirabilis plants that Correns worked with came in three types: pure green, pure white, or variegated (mottled green and white). Green and white branches could appear on variegated plants, but variegated branches did not appear on green or white plants6^6.
Correns was curious about this coloration trait, and he carried out a number of crosses between plants of different colors. He found that6^6:
  • The color of the egg cell-donating branch (female parent) determined the color of the offspring.
  • Female parent branches that were pure green or pure white produced only pure green or pure white offspring, respectively.
  • Female parent branches that were variegated could produce all three types of offspring, but not in any predictable ratios.
Correns speculated that some factor in the cytoplasm of the egg cell must determine the color of the offspring. It was actually a different German botanist, Erwin Baur, who suggested that the chloroplasts in the cytoplasm might carry hereditary factors (genes)5^5.
Baur thought that, in variegated plants, some of the chloroplasts must have mutations that made them unable to turn green (produce pigment). Today, we know that this hypothesis was exactly right!

Explaining Correns' results

How can the idea of chloroplast inheritance make variegated plants variegated? Let's follow a zygote (1-celled embryo) with mixture of chloroplasts inherited from the egg cell. Some of the chloroplasts are green, while others are white. As the zygote undergoes many rounds of mitosis to form an embryo and then a plant, the chloroplasts also divide and are distributed randomly to daughter cells at each division.
Image showing cytoplasmic segregation of chloroplasts in a plant originating from a zygote with a mixture of white (nonfunctional, mutant) chloroplasts and green (functional, normal) chloroplasts. After many mitotic divisions in which the chloroplasts replicate and are partitioned randomly, some cells will have only green chloroplasts, others will have only white chloroplasts, and others yet will continue to have a mix. The cells with only white chloroplasts will give rise to pure white branches, and the cells with only green chloroplasts will give rise to pure green branches. The cells with a mixture of chloroplasts will give rise to variegated branches, in which ongoing random segregation of chloroplasts will produce white sectors (progeny of cells with only white chloroplasts) and green sectors (progeny of cells with mixed or green-only chloroplasts). The green cells that contain a mixture of chloroplasts will continue producing occasional pure white and pure green sectors as they divide more.
Based on similar diagram found in Griffiths et al.7^7.
Over the many cell divisions, some cells will end up with a pure set of normal chloroplasts, making green patches). Others will get a pure set of nonfunctional chloroplasts (making white patches). Others yet will have a mix of normal and nonfunctional chloroplasts, producing green patches that may give rise to pure green or pure white sectors7^7.
What about the maternal pattern of inheritance? Plants make germ cells late in development, converting cells at the tip of a branch into gamete-producing cells. A branch that’s pure green will make egg cells with green chloroplasts that give rise to pure green offspring. Similarly, a branch that's pure white will make egg cells that contain only white chloroplasts and will give rise to pure white offspring.
If a branch is variegated, it has a mixture of cells, some with only functional chloroplasts, some with only nonfunctional chloroplasts, and some with a mixture of chloroplasts. All three of these cell types may give rise to egg cells, leading to the green offspring, white offspring, and variegated offspring in unpredictable ratios6,7^{6,7}.
Female branchEgg cellsZygotesOffspring
Variegated branchEgg cell with green chloroplasts, egg cell with white chloroplasts, or egg cell with mixed chloroplastsEgg cell with white chloroplasts leads to zygote with white chloroplasts; egg cell with green chloroplasts leads to zygote with green chloroplasts; egg cell with mixture of chloroplasts leads to zygote with mixture of chloroplastsVariegated plant
Based on similar diagram found in Griffiths et al.7^7.

Mitochondrial inheritance

Mitochondria, like chloroplasts, tend to be inherited from just one parent or the other (or at least, to be unequally inherited from the two parents)4^4. In the case of humans, it is the mother who contributes mitochondria to the zygote, or one-celled embryo, by way of the egg's cytoplasm. Sperm do contain mitochondria, but they are not usually inherited by the zygote. There has been a reported case of paternal inheritance of mitochondria in a human, but this is extremely rare8^8.
Both the sperm cell and the egg cell contain mitochondria and nuclear DNA. When they combine in fertilization, the resulting zygote will contain nuclear DNA from both parents, but it will contain mitochondria (and thus, mitochondrial DNA) from the egg cell only.

Maternal inheritance of mitochondria in humans

Because mitochondria are inherited from a person's mother, they provide a way to trace matrilineal ancestry (line of descent through an unbroken chain of female ancestors).
To understand how mitochondria connect you to your mother's mother's foremothers, consider where your mitochondria came from. They were received from your mother, in the cytoplasm of the egg cell that gave rise to you. Where did your mother get her own mitochondria? From her mother, that is, your maternal grandmother9^9.
If you keep asking this question, you can walk backward in time through your family tree, following your matrilineal ancestors and tracing the transmission route of your mitochondrial DNA.
Nuclear DNA is inherited from all ancestors. Over three generations, pairs of ancestors have children, leading to a single present-day person who contains nuclear DNA from eight ancestors in the great-grandparent generation, four ancestors in the grandparent generation, and two ancestors in the parent generation.
Mitochondrial DNA is inherited from a single lineage. Over three generations, pairs of ancestors have children, leading to a single present-day person who contains nuclear DNA from eight ancestors in the great-grandparent generation, four ancestors in the grandparent generation, and two ancestors in the parent generation. Just one woman in each generation is the mitochondrial ancestor of the present-day person: his mother (parent generation), his mother's mother (grandparent generation), and his mother's mother's mother (great-grandparent generation).
_Image credit: "Mitochondrial DNA vs. nuclear DNA," by University of California Museum of Paleontology (CC BY-SA 3.0)._
As shown in the diagram above, the inheritance pattern of mitochondrial DNA is different from that of nuclear DNA. A person's nuclear DNA is a "patchwork" of segments inherited from many different ancestors, while a person's mitochondrial DNA is inherited through a single, unbroken line of female ancestors9,10^{9,10}.

Mitochondrial mutations and human disease

Mutations in mitochondrial DNA can lead to human genetic disorders. For example, large deletions in mitochondrial DNA cause a condition called Kearns-Sayre syndrome. These deletions keep the mitochondria from doing their job of extracting energy. Kearns-Sayre syndrome can cause symptoms such as weakness of the muscles, including those that control eyelid and eye movement, as well as degeneration of the retina and development of heart disease11,12^{11,12}.
Genetic disorders caused by mitochondrial mutations are not transmitted from fathers to children, because mitochondria are provided only by the mother. Instead, they are transmitted from mothers to children in one of the following ways13^{13}:
  • A person with a disease caused by a mitochondrial mutation may lack normal mitochondria (and have only abnormal, mutation-bearing ones). In this case, an affected mother will always pass on mutation-bearing mitochondria to her children.
  • A mitochondrial disorder may occur when a person has a mix of normal and abnormal mitochondria her body. In this case, normal and mutation-bearing mitochondria may go randomly into eggs during meiosis. Children who get a large proportion of mutant mitochondria may have severe disease, while those with few mutant mitochondria may have mild or no disease13^{13}.
Diagram showing inheritance patterns of disorders caused by mutations in mitochondrial DNA.
Affected father and unaffected mother produce only unaffected children.
Unaffected father and affected mother with uniformly abnormal (mutation-bearing) mitochondria produce only affected children (assuming complete penetrance of the disorder).
Unaffected father and affected mother with a mixture of abnormal (mutation-bearing) and normal mitochondria may produce children with a range of phenotypes, from unaffected to mildly affected to more severely affected. these different phenotypes reflect inheritance of varying proportions of normal and abnormal mitochondria.
Image modified from "Mitochondrial," by the National Institutes of Health (public domain).
This article is licensed under a CC BY-NC-SA 4.0 license.

Works cited:

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