The Role of Mitochondrial DNA in 3-Parent IVF Babies: What Does It Mean for DNA Testing and Forensics?

Recently, 8 babies conceived through mitochondrial replacement therapy (MRT)—popularly known as “three-parent IVF”—made international headlines. Using DNA from three people, this assisted reproductive technology was developed to prevent the transmission of devastating mitochondrial diseases encoded in mitochondrial DNA (mtDNA). But what happens when these children grow up and their DNA is tested—for ancestry, genealogy, or even forensic identification?

As a mitochondrial DNA (mtDNA) expert and forensic scientist, I’ve spent my career tracing maternal lineages in modern casework and historical remains. MRT babies present a fascinating case study in how medical innovation may quietly disrupt assumptions in DNA testing.

How Mitochondrial DNA Enables "Three-Parent Babies"

The IVF method used in these cases is called pronuclear transfer (PNT). Here’s how it works: an egg from a healthy donor provides healthy mtDNA, the chromosomal DNA in the nucleus is removed, and then the nuclear DNA from the parents is transferred into the donor’s enucleated or "nucleus-lacking" egg. The resulting embryo carries nuclear DNA from the intended parents and mtDNA from the donor—hence the nickname “three-parent baby.”

AI-generated image of Pronuclear Transfer, showing cytoplasm of egg cells with unhealthy mtDNA (red) and healthy mtDNA (green).

In the U.K., MRT is offered to women with homoplasmic (100%) or high-level heteroplasmic (>~75%) mtDNA mutations—meaning nearly all of their mtDNA sequences have disease-causing (AKA pathogenic) variants. The eight babies born under the mitochondrial donation program had maternal mtDNA mutations linked to conditions like Leber Hereditary Optic Neuropathy (LHON) and rare tRNA-related disorders causing a range of symptoms including vision loss, myopathy, cardiomyopathy, hearing loss, and neurological defects.

But... Some Children Still Carried Low-Levels of the mother's unhealthy mtDNA

Ideally, all the unhealthy maternal mtDNA with pathogenic variants is excluded during the PNT process. But in 3 of the 8 cases, researchers detected small amounts of maternal mtDNA carried over into the donor egg. Human egg cells typically contain ~200,000 copies of mtDNA, and the transfer process can inadvertently bring along a few mtDNA molecules in the mother's cytoplasm. Therefore, in blood samples from the 3 infants, low levels of the mother's pathogenic variants (5%–16%) were detected using quantitative pyrosequencing. This mixture of the mother's mtDNA and the donor's mtDNA resulted in heteroplasmy, where two mitochondrial lineages coexist within the same individual. The good news: since mitochondrial diseases are dose-dependent, the low levels of the mother's pathogenic mtDNA resulted in all eight babies being healthy at birth and continuing to show normal development, according to recent follow-up studies.

Implications of Mitochondrial Replacement Therapy on Genetic Testing Results

As the MRT children grow up, many will undergo genetic testing—voluntarily or otherwise. Here’s what that DNA testing will show:

1. Maternal Lineage Breaks

Most mtDNA testing assumes maternal inheritance. In MRT cases, the mtDNA originates from a third-party donor, not the biological mother. This changes the maternal mtDNA lineage of the MRT individual, impacting genetic mtDNA ancestry results and lineage tracing for genetic genealogy.

2. Apparent Mixtures in mtDNA

In individuals with low-level heteroplasmy, labs may observe a mixture of two mtDNA profiles (AKA haplotypes) in MRT individuals. This could be mistaken for contamination, sample mixing, or the presence of multiple contributors.

What This Means for the Future of Forensic DNA Testing

As MRT and germline editing advance—this calls us to question what makes us unique in terms of our DNA. Forensic geneticists have long focused on "junk" DNA—noncoding markers that do not impact disease states. These forensic DNA markers are unlikely to be written out of someone's genetic code through genetic editing. But mitochondrial DNA can now be wholly replaced as a method of disease prevention. This calls us to question whether our mitochondrial DNA belongs to us at all…