Mitochondrial DNA vs Nuclear DNA: Differences and Inheritance

Almost all of your DNA sits in the nucleus of your cells, packed into 46 chromosomes that you inherit from both parents. But each cell also holds many copies of a second, tiny genome inside its mitochondria, the structures that turn food and oxygen into usable energy. Mitochondrial DNA (mtDNA) is a small ring of just 16,569 DNA letters, it is passed down almost entirely from your mother, and it has a fascinating history: it is what remains of a bacterium that moved into our ancestors’ cells more than a billion years ago.

Key points

Human mtDNA is a circle of 16,569 base pairs carrying 37 genes; the nuclear genome has about 3 billion base pairs (Anderson et al., 1981; Nurk et al., 2022).
Mitochondria need both genomes: 13 of their proteins are made from mtDNA and the other 99% from nuclear genes (Rath et al., 2021).
You inherit mtDNA from your mother. Sperm mitochondria lack intact mtDNA, and apparent cases of paternal inheritance are almost always explained by other DNA (Lee et al., 2023; Wei et al., 2020).
Many people carry a mix of slightly different mtDNA copies (heteroplasmy). About 1 in 200 people carry a mtDNA variant that can cause disease (Elliott et al., 2008).
An mtDNA ancestry test follows only your direct maternal line: one ancestor in each generation.

Mitochondrial vs nuclear DNA at a glance

	Mitochondrial DNA	Nuclear DNA
Where	Mitochondria	Nucleus
Size	16,569 base pairs	~3 billion base pairs
Genes	37 (13 make proteins)	~20,000 protein-coding
Shape	Small circle, many copies	46 linear chromosomes
From	Mother only	Both parents
Recombines	No	Yes
Mutation rate	Higher	Lower
Genetic code	Slightly different	Standard

Sources: genome sizes from Anderson et al. (1981) and Nurk et al. (2022); mutation rate from Stewart and Chinnery (2015); genetic code from Barrell et al. (1979). In mitochondria, for example, UGA codes for the amino acid tryptophan instead of “stop”. You can compare the mitochondrial and standard genetic codes side by side with our codon table tool.

Why the two genomes need each other

mtDNA is remarkably compact: its genes sit almost back to back with little or no space in between (Anderson et al., 1981). Yet it cannot run a mitochondrion on its own. Of the 1,136 human genes now known to make mitochondrial proteins, only 13 are in mtDNA; the rest are in the nucleus, and their proteins are imported into the mitochondria (Rath et al., 2021).

That is why mitochondrial disease can come from faults in either genome. In one large study of adults in the North East of England, disease-causing mtDNA mutations affected at least 1 in 5,000 adults, and mutations in nuclear genes caused mitochondrial disease in a further 2.9 per 100,000 adults (Gorman et al., 2015). Our guide to mitochondrial diseases covers the symptoms, diagnosis and treatment.

Where mitochondria came from

Mitochondria carry their own DNA because they were once free-living bacteria. All mitochondria descend from a single event in which a bacterium related to today’s alphaproteobacteria was taken up by a host cell related to a group of microbes called Asgard archaea. Over time, most of the bacterium’s genes moved into the host’s nucleus or were lost, leaving the small genome we see today (Roger et al., 2017).

That history explains mtDNA’s bacterial features: a circular genome, its own protein-making machinery and its own slightly different genetic code.

Why you inherit mtDNA from your mother

An egg contributes its mitochondria to the embryo; a sperm contributes almost none that matter. Sperm mitochondria do enter the egg at fertilization, but research published in 2023 found that they carry no intact mtDNA, because the protein needed to protect and maintain mtDNA (TFAM) is redirected away from sperm mitochondria while sperm are being made (Lee et al., 2023).

In practice this means:

A mother passes her mtDNA to all of her children, sons and daughters alike.
Only daughters pass it on to the next generation. Men have mtDNA in every cell, but it stops with them.
Your mtDNA is essentially the same as your mother’s, your maternal grandmother’s, and so on back along the female line.

What about reports of paternal mtDNA?

There are a few. In 2002 doctors described a man with a muscle disease whose muscle mtDNA came mostly from his father (Schwartz and Vissing, 2002), and in 2018 researchers reported three families who appeared to pass mtDNA through fathers (Luo et al., 2018). A 2020 study then found the same pattern in 7 of 11,035 families, but in every case the “paternal mtDNA” was actually a large piece of mtDNA-like sequence sitting in the father’s nuclear DNA (a “NUMT”) and inherited like any other nuclear gene. The authors found no evidence of true paternal mtDNA transmission (Wei et al., 2020). Maternal inheritance remains the rule.

Heteroplasmy: when your cells carry a mix

Because each cell holds many copies of mtDNA, they are not always identical. Having more than one version is called heteroplasmy; having just one is homoplasmy. Heteroplasmy is common: in a large genome study, about 45% of the people in 1,526 mother–child pairs carried a detectable mix (Wei et al., 2019).

The proportion of a variant can change sharply from mother to child. During egg development, only a small sample of mtDNA copies is passed on, a “bottleneck” that explains how a variant at a low level in a mother can reach a high level in her child, or almost disappear (Stewart and Chinnery, 2015). For harmful variants, this matters because they usually cause problems only once they make up a large share of the mtDNA in a tissue. That is why siblings in the same family can be affected very differently.

Harmful variants are more common than many people think. Screening of 3,168 newborns found that at least 1 in 200 carried a disease-causing mtDNA variant, most often m.3243A>G, the change behind MELAS syndrome (Elliott et al., 2008).

mtDNA and ancestry testing

Because mtDNA passes unchanged down the female line, apart from the occasional new mutation, it is a powerful tool for tracing maternal ancestry.

What an mtDNA test can tell you

Your maternal haplogroup: the branch of the global mtDNA family tree your maternal line belongs to, which points to where that line came from thousands of years ago (van Oven and Kayser, 2009).
Deep human history: a classic 1987 study traced all the mtDNA samples it studied back to one woman, probably in Africa, about 200,000 years ago, nicknamed “Mitochondrial Eve” (Cann et al., 1987). She was not the only woman alive then, just the one whose maternal line survives in everyone today.
Identity: in 2014, the remains of King Richard III were identified partly through a perfect mtDNA match with a living relative on his maternal line (King et al., 2014).

What it cannot tell you

Most of your ancestry. Go back 10 generations and you have up to 1,024 ancestors; mtDNA follows just one of them, your mother’s mother’s mother, and so on.
Ethnicity percentages. These come from autosomal (nuclear) DNA tests, not mtDNA.
Your father’s line. That needs a Y-DNA test, and only men carry a Y chromosome.

One caution: a full mtDNA sequence can reveal health-related variants. Given that about 1 in 200 people carry a potentially harmful mtDNA variant (Elliott et al., 2008), it is worth knowing how a test company reports these, and talking to a genetic counselor if something unexpected comes up.

A geneticist’s view: I love mtDNA for what it shows about human history, but I often have to gently correct what people expect from an mtDNA ancestry test. It tells a true story about one line of your family, your mother’s mother’s line, not about who you are overall. Read your haplogroup as one thread in a very large tapestry. – Dr. Sumeet Kumar

Frequently asked questions

Is mitochondrial DNA inherited only from the mother?

Yes, for all practical purposes. Sperm mitochondria lack intact mtDNA, and the rare reported exceptions have mostly turned out to be mtDNA-like sequences in the father’s nuclear DNA.

Do men have mitochondrial DNA?

Yes. Every cell in a man’s body has mtDNA from his mother. He just does not pass it on to his children.

How many genes are in mitochondrial DNA?

37: 13 that code for proteins of the energy-making machinery, 22 transfer RNAs and 2 ribosomal RNAs.

Is mitochondrial DNA more likely to mutate than nuclear DNA?

Yes. mtDNA has a higher mutation rate than nuclear DNA, which is one reason it is useful for tracing maternal lineages over thousands of years.

What is the difference between an mtDNA test and a Y-DNA test?

An mtDNA test follows your mother’s direct maternal line and can be taken by anyone. A Y-DNA test follows the father’s direct paternal line and can only be taken by people with a Y chromosome.

References

Dr. Sumeet Kumar

Dr. Sumeet Kumar is a human geneticist and the founder of GenesWellness. He earned his PhD in human genetics from the University of Delhi in 2021, with research on the genetics of Parkinson’s disease, and his work is published in peer-reviewed journals. He has been quoted as a genetics expert by Medical News Today and Salon. At GenesWellness he builds the site’s genetics calculators and explains, in plain language, how genes shape health and family traits.

Mitochondrial DNA vs Nuclear DNA: Differences, Inheritance and Ancestry