What is the status now of the gene in your view?
Denis Noble: First of all, I go along largely with Jim Shapiro’s view of the difficulty of the definition of a gene. I think it’s actually even more difficult than Jim says. My argument is very simple. Wilhelm Johannsen in 1909 introduced the definition of “gene.” He was the first person to use that word, although he was introducing a concept that existed ever since Mendel. What he was actually referring to was a phenotype trait, not a piece of DNA. He didn’t know about DNA in those days. We now define a gene, when we attempt to define it, as a particular sequence with “start” and “stop” codons, etc., in a strip of DNA. My point is that the first definition of a gene — Johansen’s definition as a trait, as an inheritable phenotype — was necessarily the cause of a phenotype, because that’s how it was defined. It was, if you like, a catch-all definition of a gene. Anything that contributed to that particular trait — inheritable, according to Mendelian laws — would be the gene, whether it is a piece of DNA or some other aspect of the functioning of the cell. That we define “gene” as a sequence of DNA becomes an empirical question, not a conceptual necessity. It becomes an empirical question whether that particular strip of DNA has a function within the phenotype. Some do and some don’t.
It’s interesting that many knockout experiments don’t actually reveal the function of the knocked-out gene. In yeast, for example, there’s a study that 80 percent of knockouts don’t have an obvious phenotypic effect until you stress the organism. What that tells me is that we have progressively moved from a definition of a gene which made it a conceptual necessity that the defined object was the cause of the phenotype — that’s how it was defined — to a matter which is an empirical discovery to be made, which is whether a particular sequence of DNA plays a functional role or not. Those are very, very different definitions of a gene.
So I go further than Jim. Not only is it difficult, as he says in his book, to now define what a gene is; one should be thinking more of networks of interactions than single and fatalistic genes at the DNA level. It’s also true that the concept of a gene has changed in a very subtle way, and in a way that makes a big difference to how the concept of a gene should be used in evolutionary biology.
This book would be a great gift for one of the many people in our lives who wonder why anyone still has any questions, once Jerry Coyne has spoken. 😉
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