Dec 05

There is no such thing as a gene

I’d like to make a few brief comments motivated by a long and very readable piece in Aeon by David Dobbs, called  Die, selfish gene, die, a criticism of the concept promoted in Richard Dawkins’s famous book The Selfish Gene.

The crucial point that needs to be made is that there is actually no such thing as a gene.  The concept of a gene was originally intended to represent the “unit” of inheritance — the minimal structure that carries meaningful genetic information.  But as genetics advanced in the mid-20th century, it became obvious that there is no such minimal unit — at least, no unit larger than a nucleotide.  The reason is recombination, an event that occurs during cell division, in which the two versions of a chromosome swap pieces.  Recombination can happen at virtually any point in the DNA.  Some points, called “hot spots”, are much more likely than others to experience a split, but as far as is known it can happen anywhere.  Thus, even disregarding mutation, there is no such thing as a chunk of DNA that is passed on indefinitely as a coherent unit.

Anybody who wants to use the word “gene” is forced to deal with that problem, but different people have dealt with it in different ways.  In particular, Dawkins in his book deals with it in a different way than the great majority of modern geneticists.  To Dawkins, as he very carefully explains, a gene is simply a stretch of contiguous DNA.  He recognizes that genes defined in this way are not distinct and don’t remain intact indefinitely, but nevertheless that is the way he defines them.  This definition makes things awkward for him in many places, but he does the best with it that he can.

Most modern geneticists, however, use the word “gene” to mean a stretch of DNA that codes for a particular end product, usually a protein.  In more technical terms, a gene in this sense is a set of exons connected by introns.  This is the usage adopted in the great majority of textbooks and journal articles, and as far as I can see it is the usage adopted by David Dobbs in this Aeon article, although he doesn’t explicitly say so.

The result is that the “selfish gene” that Dobbs thinks should die is not the same thing as the selfish gene that Dawkins was talking about.  Dobbs argues that the selfish gene concept is wrong because it does not take into account factors that alter gene expression.  But what controls gene expression?  Answer:  genes.  The factors that control gene expression are encoded by DNA, just like every other aspect of the body.  Thus, using Dawkin’s definition, they are encoded by genes.  To Dawkins, gene expression control is simply a case of genes being selfish by bossing around other genes.

My personal view is that the concept of a “selfish gene” is ultimately misleading for a different reason than Dobbs gives.  The basic problem with the metaphor is that DNA is not an agent.  It is simply a body of passive data.  A chromosome is a sort of data tape that is accessed and manipulated by a variety of agents, including proteins and RNA.  But it is not an agent itself, and can’t take actions.  If it can’t take actions, then it can’t really be selfish.

I think there is a high probability that Dawkins would actually agree with that criticism. I wouldn’t be surprised to find that he has already said so.

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