Read more: “Rewriting the textbooks: When science gets it wrong“
As Gregor Mendel showed in painstaking experiments on peas in the 19th century, many traits of living things are all or nothing. Seeds are either green or yellow, round or wrinkled, and so on. This led to the idea that an organism’s characteristics are determined by discrete “particles” passed from one generation to the next: genes.
But what is a gene? This question seemed to be settled with the discovery of the function of DNA in the 1950s. A gene, biologists agreed, was a DNA sequence that encoded the instructions for making a protein, the molecules that do all the work in living things.
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Half a century on, such harmony has vanished. We now know that a single “gene” can consist of dozens of distinct DNA segments that can be combined to make thousands of different proteins; that overlapping DNA sequences can encode quite distinct proteins; and that a few proteins are encoded by combining pieces of what were regarded as separate genes.
Even more confusingly, we are discovering ever more DNA sequences that are not blueprints for making proteins, but instead code for RNA molecules that carry out various functions directly. “If you open the door to RNA, it gets much more complicated,” says , a bioinformatics researcher at Yale University.
Reverting to an updated version of the original idea, and defining a gene simply as a DNA segment that affects the characteristics of offspring – by whatever means – doesn’t help. That’s because it would mean the inclusion not just of protein or RNA-encoding DNA segments, but also a myriad of regulatory DNA sequences that switch those segments’ activity on or off.
These days, then, what a gene is depends on who you ask. Gerstein has suggested it be defined, in simplified terms, as a union of sequences that encodes one or more “functional products”. But he readily admits this is a fudge. “What is function?” he asks. “What does it mean?” A gene that is important for survival in one species may have become redundant in a closely related strain, for instance, even though the sequence is identical. Does that make it a gene in one species and not in the other?
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