True or false: you inherit your genes from your parents.
Mostly true, but not quite. In theory, you do indeed get half of your DNA from your mother and half from your father; but in practice, there’s sometimes a third parent as well, random chance. Genes don’t always get transmitted as they should: mutations occur.
As a result, it’s not true that “genetic” always implies “inherited”. A disease, for example, could be entirely genetic, and almost never inherited. Down’s syndrome is the textbook example, but it’s something of a special case and until recently, it was widely assumed that most disease risk genes were inherited.
Yet recent evidence suggests that many cases of neurological and psychiatric disorders are caused by uninherited, de novo mutation events. Here are two papers from the last few weeks about schizophrenia(1,2) – but the story looks similar for autism, intellectual disabilities, some forms of epilepsy, ADHD, and others. Indeed they’re often the same mutations.
Biologically, a given mutation is what it is, whether it’s de novo or inherited. But on a social and a psychological level, I think there are crucial differences, and in particular I think that if it turns out that de novo mutations are important in disease, we’re going to see attempts to take these variants out of circulation – far more so than in the case of the very same genes, were they inherited.
The old eugenics movement was based on the idea that if we stop people with bad genes from breeding – by sterilization, voluntary or otherwise, say – we’ll be able to eliminate diseases and other undesirable traits. This idea is now generally regarded as extremely unethical, but many of its opponents have shared with the eugenicists the belief that it could work.
But if de novo mutations are what cause the majority of disease, then this approach would be pointless. Sterilizing certain people, or encouraging the healthy ones to have more children, would never be able to eliminate the ‘bad genes’ because new ones are being created every generation, pretty much at random.
So the de novo paradigm ought to be welcomed by opponents of eugenics. It wasn’t just morally wrong – it was biologically misguided too.
But hang on. This is the 21st century. We have in vitro fertilization (IVF), and you can analyze the genes of an IVF embryo before you decide to make it into a child. In the near future, we might be able to routinely sequence the genome of any unborn child shortly after conception.
From there, it would be a small step to allowing parents to decide not to have children with de novo mutations.
This would be, in its effects, a form of eugenics – in the sense that it would produce the effect that the old eugenicists wanted. No more ‘bad’ variants, or not nearly as many. Opinions will differ as to whether it’s morally different. But I would have said that politically, it’s a lot more likely to happen.
I can’t see forced sterilization returning any time soon. But if you were expecting a baby and you knew that it was not just carrying your and your partner’s DNA, but had also suffered a mutation – might you not want to avoid that?
Psychologically, it matters that it did not inherit the variants. It would be a big step to decide that your child should not inherit part of your own DNA. Of course, some variants are obviously harmful, like one that raises the risk of cancer, and I can’t see how anyone would want to pass those on. But think about the grey areas – a variant for social anxiety, mild autistic symptoms, obesity, a personality trait.
You might well feel that carrying that variant is what makes you, you; and so it would be natural for your child to have it. You might decide that if it was good enough for you (and all your ancestors), it’s good enough for your children. You might well resent the very idea that it’s a ‘bad gene’ at all, as an attack on your own self-worth.
But none of that applies if it’s a de novo mutation. Indeed, quite the opposite – all those same considerations would probably lead you to want your children to carry as close as possible to a carbon copy of your DNA, with no random changes. It was good enough for you.
My point is that I think there will be much more support for the idea of genetic screening or other action against de novo variants than against inherited ones. More people will want it, it will be more socially acceptable, and used more widely. I’m not saying this would be a good or a bad thing, just making a prediction. In the future, diseases and traits that are primarily caused by de novo mutations will increasingly selected against.