You inherit mitochondrial DNA only from your mother because the embryo actively destroys the father’s mitochondria after fertilization. Sperm do deliver mitochondria into the egg. They are then tagged for disposal and digested within days, leaving the mother’s set as the only one that survives.

It is one of the few genuinely asymmetric things in human inheritance. Nuclear DNA arrives in roughly equal halves from both parents. The small separate genome inside your mitochondria does not.

Both Parents Supply Mitochondria, Only One Set Survives

Mitochondria are the structures that convert energy from food into a form cells can use. They carry a small amount of DNA of their own, about 16,500 base pairs holding 37 genes, all of them essential for mitochondria to work.

Egg and sperm cells both contain mitochondria. The common explanation, that the sperm simply leaves its mitochondria behind at the egg’s surface, is not what happens. They get in. What follows is deliberate removal, not accidental exclusion.

The Embryo Actively Destroys the Father’s Mitochondria

The process is mitophagy, a targeted form of cellular recycling. Paternal mitochondria are enclosed by structures called autophagosomes and delivered to lysosomes, the compartments where cells break material down.

Work published in eLife in 2016 by Rojansky and colleagues, using mouse embryos, identified two proteins doing the tagging: PARKIN and MUL1. They work redundantly, which is why the mechanism had been hard to pin down. Removing either one alone made little difference. Removing both substantially reduced the ubiquitination that marks mitochondria for destruction, and more than 60 percent of those embryos still carried paternal mitochondria 84 hours after fertilization.

Several other components of the autophagy machinery proved necessary too, including P62, PINK1, FIS1 and TBC1D15. Knocking any of them down strongly suppressed the loss of paternal mitochondria.

How the Embryo Tells Them Apart

This is the elegant part of the finding. The cell does not appear to read a paternal label. It reads condition.

In the same experiments, paternal mitochondria gradually lost their membrane potential, the electrical charge across the mitochondrial membrane that signals a working organelle, between 18 and 48 hours after fertilization. Maternal mitochondria held theirs throughout.

Loss of membrane potential is the standard signal that marks any mitochondrion as damaged and due for recycling. The embryo is running ordinary quality control. The father’s mitochondria fail it.

Why a Single Source May Be Worth Enforcing

Carrying two populations of mitochondrial DNA is called heteroplasmy, and the working hypothesis is that it causes trouble. Mixed populations can compete within cells, and a cell that has to run two slightly different sets of energy machinery may run neither well.

The evidence here is thinner than the mechanism, and researchers say so. The link between heteroplasmy and physiological problems remains partly speculative. What is clear is that the destruction machinery is elaborate, redundant and conserved, which is not what evolution builds for something that does not matter.

What Maternal-Only Inheritance Makes Possible

The practical consequences are large, and they follow from two properties: a single parental source, and very slow change between generations.

Together those make mitochondrial DNA a clean marker of maternal lineage across enormous spans of time. Researchers have used it to trace maternal lines back hundreds of thousands of years and to study how early human populations spread after leaving Africa.

It also identifies human remains when no sample from the person exists. Because every maternal relative carries essentially the same sequence, a living descendant can confirm an identity. That approach identified the remains of the Russian Romanov family decades after their deaths, and it is the standard tool in archaeology and forensics when remains are old or degraded.

A Note on the Word Only

Reference sources phrase this carefully. MedlinePlus and the National Institute of General Medical Sciences say mitochondrial DNA is usually inherited from the mother, and that sperm mitochondria are almost always destroyed.

That hedging is deliberate. The system is a biological process with a failure rate rather than an absolute law, which is exactly what the mouse experiments show when the machinery is disabled. For genealogy, forensics and population genetics the maternal rule holds well enough to build on. As a statement of biology, usually is the accurate word.