Posted: Dec 19, 2010 11:03 am
by MedGen
Not in my Genes! - A common misconception in human genetics

Setting the scene

In Andrew Nicol’s 1997 film Gattaca the protagonist sets the scene of a dystopian society ruled by genetic determinism. What makes the film so terrifying to many is the beginning sequence that describes this none-too-pleasant world as “...the very near future...” suggesting the hard reality of a world ruled, and our lives defined, by our genomes.
Vincent, Ethan Hawke’s character, we soon find out, is part of an underclass of citizens defined not by their socioeconomic status, but by the very chemical they owe their existence too. As our hero states with frightening indifference, they have discrimination down to a science.

Gattaca shocked audiences when it was released and prompted many to question the ethics of human genetics research. I aim to show why those fears are misplaced, and dispel the myth of genetic determinism, including its cousin, the common misconception known as “The Gene for X”. Hopefully along the way I will be able to shine a little light on the beautifully complex world of how our genomes shape our very form, and our fates...

I often hear or read about genetic discoveries in the media accompanied by the catchphrase “scientists have discovered the gene for [X]”. In fact just whilst writing this I’ve heard this same old phrase trotted out with latest research on a gene called DRD4 and an apparent link with promiscuity in human adults1. So apparently, and this is only according to the media I must stress, if you have a particular variant of this gene you are more likely to have problems with fidelity! Cue the many claims by less-than-faithful partners “but my genes made me do it!” It is this kind of reporting that misleads the public about how our genomes really work. Worryingly it is not just the public that are suckered in by this grossly simplistic view, many practising doctors have only a rudimentary understanding of how our genetic make-up really works, and are thus likely to pass on this mis-information to their patients.

The bottom line is that our genetic constitution does not define us; it influences us to varying degrees. At this juncture I must point out that this applies to all aspects of genetics, including what are called Mendelian disorders; the classically defined genetic diseases such as cystic fibrosis and sickle cell anaemia. I know what you’re thinking – “He’s lost the plot! I know that genetic mutations cause these diseases!” and of course you would be correct, however, there is a very important, if somewhat subtle detail that must be taken into account. A little background in molecular biology will hopefully shed some light on this conundrum.

Genetics 101

There is phenomenon or concept, if you will, in molecular biology that explains how information encoded in a DNA molecule is translated to a functional protein. It is proteins that do most of the work in our cells and bodies. They turn genes on, and off, they help us breakdown foodstuffs and extract energy from what we digest, as well as controlling various aspects of our immune systems and how we grow as a foetus. Needless to say they are rather important. This phenomenon is known as the central dogma, and it states that information flows from DNA to a related molecule called RNA, before it is finally translated into a sequence of amino acids which are the building blocks of proteins. So we can say that the passage of information follows this sequence:


Whilst there are some important exceptions to this rule, information does not flow the opposite way from a protein to create a new DNA sequence. This is very important because if we change information in the DNA molecule that encodes the protein (i.e. a gene) we can change a part of the protein itself. Et voila! A mutation.

Mutations can occur in all sorts of places in DNA, and can subsequently have all sorts of different effects on not just the protein itself; it can change the shape of the protein, and thus how it works, or it can stop it from interacting with other molecules, or it can give it a new function entirely. As a result a mutation can cause catastrophic damage to our cells, and our bodies, leading to a genetic disease such as cystic fibrosis or sickle cell anaemia. What is very important here is to understand that each protein does not work in isolation; it fits into a very complex machine with hundreds, often thousands, of finely tuned interacting components. Much like a watch that relies on a number of different cogs turning at different rates, thus a cellular network of proteins relies on the careful regulation of the function of interacting networks of protein molecules. The area of biology tasked with understanding these interacting networks is termed systems biology, and is a relatively new player on the field of molecular biology. Previously scientists, and in fact they still do, study proteins and biological molecules in isolation, an approach called reductionism. This approach allows scientists to break apart complex problems and study them a little bit at a time. Unfortunately it this approach that has had a knock-on effect; our interpretations are also affected by this reductionist methodology.

So what has this got to do with genetics and genetic diseases? A few cogent examples may help to explain this. There are many mutations in a particular gene that cause cystic fibrosis (CF); the cystic fibrosis transmembrane conductance regulator, CFTR. Cystic fibrosis, like any disease, is a collection of phenotypes (symptoms) that manifest themselves together in a person because of a common cause, in this case a mutation in our gene CFTR. This is where we need to be careful; this gene does not cause CF. Specific mutations within this gene lead to the formation of multiple phenotypes that co-segregate together. That doesn’t mean necessarily that any mutation within this gene will always cause CF, in fact some mutations lead to a related, but less severe condition called chronic bilateral absence of the vas deferens (but only in males of course!). We find a similar scenario in sickle cell anaemia. Mutations in one of the proteins that make up haemoglobin, the protein that is responsible for carrying oxygen in red blood cells so that it can be fed to all of the tissues of our bodies, leads to the formation of stiff fibres in red blood cells that causes them to take on a characteristic “sickle” shape. These sickled cells do not carry oxygen to tissues and just clog up small capillaries due to their inflexibility. These cells get damaged and die, and also rupture small blood vessels, the result of which is anaemia and chronic internal bleeding. This condition is not the same in everyone though. There are a number of different factors that can affect how severe the disease becomes, some of which may also be genetic, but importantly some of these can be environmental.

In both of these scenarios mutations can be associated with the disease as a whole, but very rarely is the phenotype mapped 1:1 with the mutation; this is one of the major difficulties of molecular pathology. Each of these mutations are generally rare (the examples of CF and sickle cell anaemia are more common in certain populations for other interesting reasons that cannot be explained here), so what about the 10 million or so common variants we all carry around with us?

There are a number of different mutations that are so common within the human gene pool (or any species gene pool for that matter) that they are instead referred to as polymorphisms (meaning many bodies). Some of these have an effect on protein function and thus the observed physiological phenotype, whilst others may affect the regulation of a gene. Importantly some of these may have no function at all, a fine example of truly neutral mutations.

From ref[2]

The diagram above illustrates the link between how common a variant is and how large the effect it has on the phenotype, called its penetrance. Mutations that have a high penetrance are those that cause Mendelian diseases like sickle cell anaemia and cystic fibrosis. If a mutation does not cause a trait to be expressed and only influences it modestly then we say it has an incomplete, or lower, penetrance.

Enter the Post-Genomics Era

In recent years a hypothesis was developed within human genetics research that perhaps it would be possible to investigate the genetic component of complex characteristics, polygenic traits that are the summation of multiple interacting genetic variants all with small individual influences on the trait of interest. This trait may be a continuous trait such as adult height, or intelligence, or it may be a binary outcome such as the occurrence of a disease such as cancer or type-2 diabetes (called adult-onset diabetes). Thanks to technological advances the tool used to investigate this hypothesis is called the genome-wide association study; the aim being to investigate as much of the genome at once to try and capture any potential influence on the trait of interest.

Thousands of these genome wide studies have been performed in the last 5 years or so attempting to discover the genetic component of a wide range of diseases and characteristics. The result has been a mountain of associated genetic variants, with very little functional consequence to compare them to. It is though that partly this may be due to type I errors, false positive results. An alternative, more...biological explanation, is that because each variant has such a small impact on the trait or disease that changes in the way they are classified will be able to highlight clearer associations.

Another major issue with this common disease-common variant hypothesis is that very little of the variation between individuals can be explained with these genetic variants alone. Some have suggested that perhaps they have very little role to play - cue the Nature vs Nurture debate. Importantly it is how our environments and genomes interact that modulates any influence either may have on the outcome of a particular characteristic.

Anything for a quick buck?

No discovery would be complete without someone trying to make a quick buck or two from it. Genetics research is no different and in recent years several companies have sprung up that will offer you a read out of parts of your genome for the modest sum of $500. These companies copped a lot of flak recently from the FDA, not because of the potential problems with interpreting the results of the tests they sell, but that they are associated with diseases and thus come under the guise of medical diagnostics which require strict FDA regulation, i.e. the FDA wanted to police them so they attempted to use the flimsiest possible reason for classifying them as diagnostic tools. As a result these direct to consumer genetic tests have had a bit of a rocky infanthood for political, not scientific, reasons.

What about the scientific reasons?

Importantly, and this has been a little overshadowed by the FDA’s heavy handedness, what these companies are offering is not so far removed from the scenario we see moments after the birth of Vincent in Gattaca. They are providing discerning customers with a prediction of their health on the basis of their genetic make-up. Whoah! Hang on! Does that mean we have already entered our Gattaca-style world without realising? In short, no. In long, what these companies provide is a very crude prediction of the impact of individual impacts on the customers health for a range of different diseases, and a number of superficial traits, such as wet or dry earwax, ability to taste a bitter chemical called phenylthiocarbamide (PTC), and whether you have curly hair or not (I’m not sure if I need a genetic test for two of those), but hey! It’s just a bit of fun right? Well, yes and no. Yes because it helps to bring the complex world of human genetics into the public domain, but also no, because people who take these tests may take the results to their local GP and find out that their knowledge of these tests is inferior to their respective Wikipedia page.

Let’s take an example. A variant within an important gene involved in the functioning of our immune systems called IL7RA has been associated with a slight increased risk of developing multiple sclerosis. Now that sounds quite serious to be at an increased risk of developing such a debilitating autoimmune condition. No need to go and get a wheel chair quite yet, put the phone down, there’s no need to book an appointment with your GP, we need to look at the level of the risk first. In the study itself people who carried two copies of this particular polymorphism were 1.08-fold more likely to develop MS compared to a population of healthy individuals. fold. The average risk of Joe Bloggs on the street is 0.70 in 100, i.e. a 0.07% lifetime chance of developing MS in the absence any other information that might increase your risk. Carrying two copies of this variant increases that lifetime risk to, wait for it...0.096%! Even these risks are dependent on your age and your ethnicity. Not all human populations even carry around the same genetic variants. Of course if you do end up developing MS you can be sure that those little mutations may, not will, have had an influence on when you develop the disease, and potentially how mild or severe it is.

This example is fairly typical of most genetic variants that affect our chances of developing a particular disease or reaching a particular height. Even if we manage to discover all of the genetic influence on diseases like MS we still won’t be able to give a 100% certitude that you will or won’t develop the disease because the genetic component probably only makes up 30-40% of the factors that influence risk of disease. Or for a continuous trait they only explain a portion of the variation between individual people.

Tying up Loose Ends

How does this relate to our “Gene for X” misconception? Hopefully the more astute amongst you will have noticed I have not said that a particular gene is the cause for any disease or trait, rather it is specific variants that we find associated with characteristics. So next time you hear that scientists have discovered the gene for coffee drinking, or some other equally inane human behaviour, take a second to recall that obscure essay you read on an internet forum once, and think about how likely it is that there is a single gene that controls whether you like your caffeine from coffee or tea.

All of our behaviours, characteristics and risks of disease are of course going to be influenced by our genetic make-up. I hope I have been able to paint a picture of how the summation of our genetic variation we carry with us plays such a major role during the course of our lives, but that it does not define who we are.


[1] Garcia et al (2010) Associations between dopamine D4 Receptor gene variation with both infidelity and sexual promiscuity. PLoS ONE 5(11): e14162. doi:10.1371/journal.pone.0014162

[2] McCarthy et al (2008) Genome-wide association studies for complex traits: consensus, uncertainty and challenges. Nat Res Genet 9, 356-369

[3] Zhang et al (2005) Two genes encoding immune-regulatory molecules (LAG3 and IL7R) confer susceptibility to multiple sclerosis. Genes Immun 6(2):145-52.

Edit: Typo correct - "differens" to "deferens"