mitochondrial transfer

My latest column for the New York Times is on the debate about ‘three parent babies’, published in the NYT under the headline ‘The Three-Parent Baby’s First Step’. Here are the opening paragraphs. You can read the full article in the NYT.

The British Parliament can be an archaic, backward-looking institution, wedded to tradition, and not known for taking a revolutionary stance. Yet its members have just made a groundbreaking decision, one that no other legislature has so far been willing to contemplate.

They approved legislation that makes Britain the first country to allow the creation of what many call ‘three-parent babies’. Supporters say the procedure will enable women to avoid passing on certain severe and even deadly genetically inherited diseases. But many regard the new law as an unwise, even immoral, move — the first step toward the creation of ‘designer babies’. Some even see it as a new experiment in eugenics.

‘Three-parent babies’ is a sensationalized term to describe a special form of in vitro fertilization, or IVF, that is better labeled ‘mitochondrial transfer’. Every human cell comprises two main parts: the nucleus and the cytoplasm. The nucleus contains the DNA, the genetic code that helps shape inherited traits. The cytoplasm is the workshop of the cell, where most day-to-day functions occur. Among its constituent parts are mitochondria, tiny organelles whose job it is to provide energy. Each mitochondrion contains tiny amounts of its own DNA, some 37 genes compared with the 20,000 or so in the nucleus. (It is thought that back in evolutionary history, a free living bacterium became trapped in a host cell, where it boosted the cell’s capacity to produce energy; over time, it evolved into an organelle, an intimate part of the cell, but retained its own DNA.)

Mitochondrial DNA plays no part in determining an individual’s inherited traits, such as those that shape appearance or talents. But if it malfunctions, it can cause terrible conditions like muscle weakness, seizures, blindness, deafness, organ failure and even death.There are more than 50 mitochondrial diseases, affecting at least one in every 8,000 children (some suggest the figure is much higher). There are currently no cures.

In mitochondrial transfer, the healthy nucleus of an egg with damaged mitochondria is transferred into the body of a healthy donor egg from which the nucleus has been removed. When that egg is fertilized, it will have its normal complement of genes from the mother and father, together with a tiny amount of mitochondrial DNA from the healthy third-party egg into which the nucleus was transferred. (The law also permits a more complex alternate technique, in which both eggs are first fertilized in vitro, and then the nucleus of the egg with defective mitochondria is transferred to the healthy donor egg, from which the nucleus has been removed.)

Hence the notion of a ‘three-parent baby’ — misleading though the term is. None of the socially or ethically important characteristics of the child will derive from the ‘third parent’. What makes you ‘you’ depends upon far more than your genes, but insofar as genes play a role in personality or talent or physical attributes, they all derive from the nucleus. In the case of mitochondrial transfer, such genes will come, not from the woman who donated the egg with working mitochondria, but from the nucleus of the woman with faulty mitochondria and from the sperm of her partner. Those two are truly the child’s only parents.

Continue reading the article in the New York Times.

Photo courtesy of the Roslin Institute.


  1. So what *is* wrong with improving a new person’s heritable traits?
    Whether that’s bringing kids up from negative traits to a neutral baseline, or indeed above that baseline to be an improved version?
    What’s so “unnatural” about selective breeding, or evolving, if you will?
    I’d be interested to hear your views on this.

  2. We went through this whole conversation with ‘test tube babies.’

    Reading the coverage at the time you’d think the streets of the 21st Century would be full of goose stepping übermensch.

    If scientists are ‘playing God’ that’s because there’s a vacancy. No God would allow a child to suffer needlessly.

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