Several Living People Now Have DNA from Three Genetic Parents

Back on November 14, 2009 we reported that scientists were inching closer to three parent babies using a technology known as mitochondrial replacement therapy (MRT). Unlike many promises still waiting to manifest after a decade or more, this one has actually panned out.

As of 2025, mitochondrial replacement therapy (MRT), often called “three-parent IVF,” has moved from early experimental research to successful clinical treatments, offering hope to families affected by devastating mitochondrial diseases. But what exactly is MRT, why is it important, and why does it spark so much controversy?

Mitochondria are tiny structures inside our cells that produce energy and have their own small set of DNA, separate from the DNA in the cell’s nucleus (which determines most of our traits). Sometimes, mutations in mitochondrial DNA cause serious, inherited diseases that affect organs like the brain, heart, and muscles, often leading to severe disability or death.

MRT is a reproductive technology designed to prevent these diseases by replacing the defective mitochondria in a mother’s egg with healthy mitochondria from a donor egg. This means the resulting child inherits nuclear DNA from both parents, but mitochondrial DNA from a third person—the donor—hence the term “three-parent baby.”

In 2025, researchers in the UK celebrated a major milestone: eight healthy babies born via MRT, all showing normal development so far. These successes followed decades of research, including early work in Japan that demonstrated damaged eggs could be “repaired” with donated mitochondria to improve fertilization.

Why should anyone care? Beyond offering a chance for affected families to have healthy biological children, MRT challenges our understanding of genetics, heredity, and identity. It raises ethical questions because it involves altering the genetic material passed down through generations—potentially affecting not just one child but their descendants forever.

Mitochondrial DNA (mtDNA) carries genes that primarily influence the function of mitochondria—tiny structures inside cells responsible for producing energy. Unlike the nuclear DNA that determines most of our inherited traits like eye color or height, mtDNA codes for components essential to cellular energy production. Here are key points about traits influenced only by mtDNA:

• Energy production: mtDNA encodes genes for proteins involved in the mitochondria’s ability to generate ATP, the cell’s main energy source. This impacts overall cellular energy metabolism.
• Subtle influences on health and physiology: Variations in mtDNA can affect susceptibility to some common diseases such as type 2 diabetes and multiple sclerosis, as well as potentially influencing lifespan, metabolism, and indirectly, some traits like height. (Height is a complex trait influenced by many factors, including nuclear genes, nutrition, environment, and cellular energy metabolism. Since mitochondria are the powerhouses of the cell providing energy, variations in mtDNA that affect mitochondrial efficiency and energy production can subtly impact overall growth and development, including height.
• No impact on visible physical traits like eye color or hair: mtDNA does not determine external features or personality traits directly; those come from nuclear DNA.

Critics worry about the long-term safety and unforeseen consequences of modifying embryos. They also raise concerns about “designer babies” and whether this technology could open the door to more controversial genetic modifications. Supporters argue that preventing serious diseases outweighs these risks, especially when carefully regulated.

Clinically, MRT is still limited to preventing mitochondrial diseases and is only legal in a few countries like the UK and Australia. Other nations, including the US, have strict restrictions, reflecting ongoing debates among scientists, ethicists, policymakers, and the public.

In short, MRT represents a powerful, life-changing medical advance combined with profound ethical challenges. It’s a striking example of how science is pushing the boundaries of what’s possible—and forcing society to ask what should be allowed when it comes to reshaping human genetics.