Crossing the germ line facing genetics' great taboo

Let's stop drawing lines in the sand when it comes to genetically modifying people and talk about engineering everybody

Want to see what a genetically modified human looks like? Just glance in the mirror. You are the result of an experiment that randomly modified your DNA in at least 50 places.

No ethics committee in the world would approve such a dangerous practice. But hey, it's OK because the scientist in this case is nature. And nature is good, right? Never mind that some unlucky kids die horrible deaths because they end up with cruel and fatal mutations. Never mind that just about every one of us will suffer at some point because of the legacy of countless generations of this uncontrolled experiment.

What if we could put a stop to this? We have already begun in a small way. For the past three decades some communities have been screening would-be parents to ensure their children do not inherit one particularly cruel genetic modification Tay-Sachs disease. More recently, we have begun to screen IVF embryos before they are implanted in cases when we know children risk inheriting one or other of the nastiest results of nature's meddling.

And now, with the UK parliament's vote in favour of three-parent babies, we are about to go a step further and actively replace damaged genes with working versions that can be passed on to subsequent generations, breaking the chain of a range of inherited diseases. Great! This form of genetic engineering should result in the birth of healthy children and end much suffering... but wait! Gasp, horror! Did I write the e-word? I'm sorry, I meant "mitochondrial donation".

The decision to allow three-parent babies is right. But the fact is, opponents were also right to describe this as a step towards tinkering with the rest of our genome. Most supporters seemed to have convinced themselves otherwise, but let's look at the arguments.

One is that mitochondrial replacement is no big deal because mitochondria contain just 37 of the 23,000 or so human genes. Sure, but most genetically modified plants and animals have only one or two altered genes. If replacing genes is OK as long as it's only a small proportion, you could justify quite substantial alterations this way.

Ah, we are told, but the point is that these 37 genes do not affect children's characters or appearance. The "only known traits" that could come from the mitochondrial DNA concern energy production, proponents of the technique have argued in New Scientist. Fine. But most of our 23,000 genes are involved in fundamental processes such as cell division, and do not have any known effects on our character. So by this logic, it is OK to tinker with most of our genes.

Of course, replacing faulty mitochondria, which are self-contained organelles within the cell, is relatively simple and we think safe. Replacing or altering genes in the cell nucleus is much trickier. It involves editing DNA by cutting and pasting bits of it recombinant DNA technology and it is not safe at the moment. It would be utterly wrong to attempt in people with existing technology.

But the technology is advancing at a breathtaking pace. We're getting much better at editing DNA, with the help of easier and more precise techniques such as CRISPR, and we can now check those changes with whole-genome sequencing. It could be just decades before it is safe to attempt germ line genetic engineering using recombinant DNA technology.

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Crossing the germ line facing genetics' great taboo