What Does a “3 Parent Baby” Look Like?

There has been some recent controversy (which promises to last for a good long time) about how the UK has voted to allow the creation of “three-person babies”, in which an embryo is created from the genetic material of three distinct individuals.

I always hate when journalism and politics meet science because it is likely that someone is about to act very stupidly. In this case, I’ve come to believe the very phrase “three person baby” is a loaded phrase that doesn’t accurately describe the reality of the procedure under question.

What, technically, makes a “three person baby”? The phrase evokes the thought of a child somehow spliced together roughshod from three disparate sources of DNA. The reality is that the “third parent” in this equation is more akin to a blood donor than an additional parent.

First, a little background. Or you can TL/DR to the visual near the bottom.

Humans have 23 chromosomes from each parent. They are numbered one through twenty-two and then XX (for females) or XY (for males). The egg and the sperm that fertilize it each have this genetic material responsible for the development of a new human being. You knew this, right? Reproduction 101.

But outside the nucleus in the egg are the mitochondria, organelles responsible for generating energy for the cell. The mitochondria has its own genetic code and, because it is in the egg, it is passed on exclusively from mother to child. After fertilization as the egg divides to develop into an embryo, each cell contains mitochondria from the original egg including, of course, that original mitochondrial DNA.

The problem lies with the fact that there are some very potent mitochondrial diseases. The “three person baby” technique is actually a rather ingenious one in which (to simplify) the nucleus of mom egg is removed from the cell with the unhealthy mitochondria and placed into a donor cell (from the “third parent”) to be fertilized by dad’s sperm.

So the only way in which it is a “three parent baby” is that it has all of mom’s chromosomes and all of dad’s chromosomes and someone else’s mitochondrial DNA.

Does this present a problem? I don’t think so.

It turns out that mitochondrial DNA is small. Not just small. Really small. Tiny.

Compare the mitochondrial DNA to human chromosome 1. Chromosome 1 has almost 250 million base pairs (the atomic unit of DNA). Mitochondrial DNA is 65,000 base pairs. I’ve seen the phrase “babies with 0.1% of their DNA from the second woman”. Not really. More like 0.002%. The percentage is actually so small that it’s hard to explain how little genetic material can be attributed to a “third parent”.

The most appropriate way to understand it is visually.

If we were to look at the DNA of a “three parent” child and showed which parts came from mom and dad and which parts came from the “third parent”, it would look like this.

The fact of the matter is that this isn’t a “three parent baby” any more than a person with a heart transplant is a Frankenstein. It’s a two parent baby with a mitochondria transplant. Just like we might transplant an organ to save a life, we’re now capable of transplanting an organelle to do the same.

This is such a revolutionary concept to our understanding of medicine that there isn’t really a good analogy to fall back on. While I’m comfortable calling this an “organelle transplant”, it is true that organ transplants do not result in the recipient passing that organ on to future generations.

That, of course, raises a curious hypothetical: If the baby is a boy, is the ethical problem solved? He can’t pass on that “third parent” DNA, so it is almost exactly like an organ transplant. But the idea that this life-saving therapy is only valid if the baby is a boy is… well, it’s a little horrifying.

The truth of the matter is that, while there is a technical element of truth in the phrase “three person baby”, it should challenge us to think about redefining what we mean by the word “person” in that phrase. Yes, it requires a third individual to donate their healthy mitochondria to stand in for mom’s unhealthy mitochondria, but are we willing to say that such a small change, equivalent to donating a single drop of blood to a fully grown man, is enough to redefine how we view a child’s genetic parentage?

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