Identical twins usually do not die from the same thing

Over at Nature News, Erika Check Hayden has a post about a recent Science Translational Medicine paper by Bert Vogelstein and colleages looking at the potential predictive power of genetics. The take-home message from the study (or at least the message that has been taken home by, e.g., this NYT article) is that DNA does not perfectly determine which disease or diseases you may get in the future. This take home message is true, and to me relatively obvious (in the same way that smoking doesn’t perfectly determine lung cancer, or body weight and dietary health doesn’t perfectly determine diabetes status).

A lot of researchers have had a pretty negative reaction to this paper (see Erika’s storify of the twitter coverage). There are lots of legitimate criticism (see Erika’s post for details), but to be honest I suspect that a lot of this is a mixture of indignation and sour grapes that this paper, a not particularly original or particularly well done attempt to answer a question that many other people have answered before, got so much press (including a feature in the NYT). A very large number of people have tried to quantify the potential predictive power of genetics for a number of years – why was there no news feature for me and Jeff, or David Clayton, or Naomi Wray and Peter Visccher, or any of the other large number of stat-gen folks who have been doing exactly these studies for years. ANGER RISING and so forth.

But of course, the reason is relatively obvious. All of the papers I linked to there are by statistical geneticists, are technical, discuss models and the merit of various predictive measures, and never came with a press release or an attempt to talk to the public about them. The message, to those who can read them, is clear and well established – genetic risk prediction (or any form of risk prediction) will never be able to perfectly predict disease incidence, and will never replace diagnostic tests. But the fact that the results of Bert Vogelstein’s study seems to have come as a surprise to people, when it comes as no surprise to us, shows us that we have failed in one of our primary duties to keep the public informed about the results of our research. The paper’s failure as a work of statistical genetics stands in contrast to its success as a work of public outreach. If we are annoyed that a bad paper got the message across, then we should be annoyed with ourselves that we never communicated our own results properly. Here is some small attempt to rectify that:

People with exactly the same weight, height, sex, race, diet, childhood infection exposures, vaccination history, family history and environmental toxin levels will usually not get the same disease. Identical twins, despite sharing the same DNA, the same socioeconomic background, the same childhood environment and (usually) the same bloody placenta, will usually not get the same disease. There is no health destiny, there is always a strong random component in anything that happens to your body. This does not mean that none of these things are important; being aware of your disease risks is one of the most important things you can do for your own future health. But risk is not destiny.

If there is one take-home message, one bite-sized bit of knowledge that everyone should know about genetic health, it is that identical twins usually do not die from the same thing, BUT that they are far more likely to than two random individuals. This is a perfect analogy for how well (and badly) risk prediction can work: you will never have a better prediction than knowing the health outcomes of essentially another copy of you*. The health outcomes of another version of you will be invaluable, and will help guide you, your doctor and the health-care establishment, if they use this information properly. But it won’t let them know exactly what will happen to you, because identical twins usually do not die from the same thing. Hold that in your head, and you will have understood about as much important information as you would have got from reading Bert Vogelstein’s paper (or any of the others I linked to). The rest is just statistical genetics, important for health-care planning and research (and for its inherent interest), but secondary to the main message.

Identical twins usually do not die from the same thing.

The image above is from Wikimedia commons.

*Sorry to MZ twins, who I guess will find that description a bit offensive. But on the other hand, you get the awesome privilege of knowing someone who is essentially a copy of you, so from where I’m standing you still come out on top.

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12 Responses to “Identical twins usually do not die from the same thing”


  • Well said against determinism. Now, if you titrated twins by # associated variants, I’d bet you’d get more concordance where a pair were at high risk. But that too, is obvious.

    My grad boss even wrote a book about this [shameless plug]: http://www.amazon.com/Genetic-Destinies-Peter-Little/dp/0198504543

  • So are you saying that if one twin is killed by lightning then it is NOT 100% certain that the other one will as well? Wow.

    All the hype and PR about this paper, it reminds me of the rant about the environment some time ago (“summarised” here http://bit.ly/HPJ4kU)

  • Also you have a reasonable point about the message and getting it out there that genetics is not deterministic for common diseases. The trouble is that many reports, eg. NYT are missing the point again and implying that personal genetics is a waste of time and money. It’s all still too polarised

  • For conjoined twins, I would say it IS 100% certain. :)

  • Bravo! This post puts the whole situation into context much more effectively than anything I’d read to this point. And you make excellent points that didn’t occur to me at first, particularly regarding communication.

    As the previous commenter said, the slant that filtered out to the media was a big disappointment. To my mind Vogelstein et al may have succeeded in reaching a wide audience, but they failed in effectively communicating to it. It’s been painful to read all the variations of “genomics is a crock” in the headlines this week.

  • I completely agree with your point about communication and outreach.

    This is another lesson in the importance of communicating our results not only effectively, but also responsibly to the public.

  • Wonderfully spoken. As someone who himself writes a blog to and tries to be a bit of a public “educator” about cancer research, there is a huge take-home message here. Thank you for posting this.

  • My mom and my aunt were identical twins. They both died in their sixties of cancer within a few years of each other (ovarian cancer and breast cancer). They lived in different countries as adults. My mother had three children and my aunt had none.

  • Many thanks for the well-reasoned commentary. Sad to see that something so relatively obvious would be reported as ‘news’, but so it goes in a world of journalism where if it doesn’t bleed, it won’t lead…

    It is a sad commentary on the current state of science reporting. The current economic pressures put expediency over any kind of attempt at informing the general public on the importance of genetic research in the current ‘century of biology’.

  • Do we currently know how many somatic mutations will occur during the rapid mitosis in early embryonic development? Is it possible that part of the ~100 new mutations per child occur in this stage, and not in the meiotic divisions that produce gametes? If so, even identical twins are not identical (or rather, they will have substantial mosaicism that may be present in most, but not all of their cells, which differentiates them from each other). In this scenario, the most recent variants in the population, which are a priori more likely to have deleterious impact, would also be differentiating the twins. Add to this the recent work on transposon reactivation during neural development, which produces unique genetic brains, even in monozygotic twins, and perhaps they are not so alike after all?

  • I am curious if anybody commmenting is an identical twin? My identical twin died of AML and so came across this article in my research for answers in our DNA. It is hard to find anybody in my situation which is why I was curious to read further on genetics (uncle is a hem/onc but it is hard to get advice from family) but also scares me because maybe there are not many left of the surviving pairs so am I rare??

  • One common fear that has concerned some is the link between genetic disease and heredity. However, epigenetics is a relatively new field that is researching external factors associated with genetic disease. Smoking, excessive light exposure and others effect how certain genes become active then cause disease. In some cases, it could be strictly genetic. However, epigenetics has added a new variable.

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