Are there 'genes for' traits?
By John Dupré (Professor of Philosophy of Science at the University of Exeter)
This article forms part of a School Resource Pack created by the Progress Educational Trust (PET) as part of its project Spectrum of Opinion: Genes. The article incorporates links to an accompanying Glossary of terms, and is followed by a list of 10 key words, phrases and names and a set of Questions to consider. A more extensive version of the article can be found on PET's BioNews website.
It is difficult to avoid media stories about 'the gene for' something, because they appear so often. On the day I wrote this article, the discovery of a 'gene for longevity' was being reported. A newspaper story suggested that lifespan is fixed at conception, contradicting stories elsewhere in the same newspaper that said that lifestyle affects health and longevity.
Are there 'genes for' some characteristics? It depends, of course, on what you mean. The well-known scientist and author Richard Dawkins, for example, is careful how he defines this expression. A 'gene for X', according to him, is any bit of DNA that makes an organism possessing it more likely to have trait 'X' than an organism with some other DNA sequence. This definition could be useful in some technical contexts, but in popular discussion it is usually misleading, for two reasons.
First, using Dawkins' definition, there are no 'genes for' traits universally possessed by a species. If a 'gene for' increases the likelihood of possessing a trait, then you encounter the problem that the likelihood of possessing a universal trait cannot be increased. So using this definition, 'genes for' traits are not responsible for building organisms. They are merely the explanations for differences between organisms.
Second, almost every trait that varies between organisms is affected by many genes. Endless confusion results when 'gene for' is used to mean any of the many genes that affect one trait, or is taken as an adequate explanation of the differences between individuals in a species.
Further confusion occurs with the tricky, but important, case of genetic influence on continuously varying traits like height or weight. There are probably many genetic and other factors (for example, diet or exercise) that contribute to how fat or thin people are. This is well known to plant and animal breeders, who try to identify genetic markers affecting the varying traits they wish to modify. But such techniques only identify genetic variants that are statistically important. They cannot tell us whether a trait is 'genetically determined'.
With so many factors affecting traits such as height or weight, a gene that satisfies Dawkins' definition of a 'gene for' weight may play only a small part in controlling a person's fatness or thinness. Furthermore, this same gene may be involved in producing many other traits. So outside of technical contexts, knowledge of this gene does little or nothing for our understanding of weight. The processes through which traits like weight develop have too many causes for anything to be a defined as the central cause.
In some instances, a single different protein may produce a superficial but recognisably different phenotype - for example, eye colour. But mainly, what we think of as genes that determine particular phenotypes are actually errors in genes, alterations that prevent the production of a particular protein. These alterations usually have pathologicalconsequences, for example conditions such as cystic fibrosis.
Just because defects in certain genes generate specific disorders in the body does not mean that the same genes control the body's healthy functioning. To take a non-biological example, if you cut the fuel line to a car, it won't start. It would be wrong to assume from this that the purpose of the fuel line is to make the car start.
The expression 'gene for' is potentially very misleading. The cases where it might be useful are very specific, and a great deal of confusion could be avoided if it were not used at all.
10 key words, phrases and names
The expression 'gene for' can be very confusing. Why is it used so often in the media?
If you want to explain how our genes relate to our phenotype, what other words and expressions might you use, instead of 'gene for'?
If an error in a particular gene results in a particular characteristic, what does this tell us about the relationship between the gene and the characteristic?