What a gene is, and why the answer keeps changing
What a gene is, and why the answer keeps changing
A-level Biology gives the gene a clean definition. The real history and current science of the gene concept are far messier, and for anyone applying to biology, biochemistry or medicine, the mess is the interesting part.
A-level Biology defines the gene tidily: a length of DNA that codes for a protein, the unit of heredity passed from parent to offspring. The definition is useful and you should know it. It is also a simplification of a concept that biologists have redefined repeatedly over a century, and that remains contested today. The neatness hides the science, and the science is what makes the subject worth pursuing beyond the syllabus, as we argue more generally in our note on revising biology so it stays put.
A concept with a history
The gene began as an abstraction. When Gregor Mendel worked out the rules of inheritance in the 1860s, he had no idea what a gene was made of. He inferred the existence of discrete hereditary factors from patterns in pea plants, decades before anyone knew that such factors had a physical form. Early twentieth-century genetics, above all Thomas Hunt Morgan's work on fruit flies, located genes on chromosomes and pictured them as beads on a string. Only after Watson and Crick described the structure of DNA in 1953 did the gene become molecular: a sequence of bases that specifies a protein, captured in the slogan of one gene, one enzyme. For a while the picture looked settled.
Why it stopped being simple
Molecular biology then complicated its own tidy account. The relationship between a stretch of DNA and a protein turned out not to be one to one. Through a process called alternative splicing, a single gene can be read in different ways to produce many distinct proteins, so the count of human proteins far exceeds the count of human genes. Large regions of the genome that make no protein at all do real regulatory work, switching other genes on and off. Some genes are transcribed into RNA that is functional in itself and never becomes a protein. Genes overlap, sit inside one another, and are read in pieces. The question of how much of the human genome is functional, brought to a head by the ENCODE project, became a genuine scientific controversy rather than a settled number. The molecular gene, so clean in 1953, had become hard to define with precision.
One word, several jobs
The heart of the matter is that the word gene is quietly doing several different jobs. It names a unit of inheritance, the thing passed between generations. It names a unit of function, the thing responsible for making a product. And it names a stretch of sequence, a physical piece of DNA. In the simple cases these coincide, which is why the A-level definition works. In the complicated cases they come apart, and no single definition captures all three at once. This is not a failure of biology. A concept can be indispensable and imprecise at the same time, and the historian of science Evelyn Fox Keller wrote a whole book on how the gene held the twentieth century together while its meaning shifted underneath it.
Levels of explanation
A related trap hides in the phrase "a gene for". Genes do not code for tallness, or blue eyes, or a disease in any direct way. They code for molecular products, which act in the context of other genes, of cells and of environments, and the trait emerges from all of that together. Richard Dawkins's account of evolution from the gene's point of view is one powerful way of explaining why genes behave as they do over evolutionary time, but it sits alongside explanations pitched at the level of the cell, the organism and the population, and a good biologist knows which level answers which question. To hold these levels apart, and to see how they connect, is a large part of thinking clearly about living things.
For applicants to biology, biochemistry or medicine, this is the kind of understanding that turns a personal statement from a list of interests into evidence of a mind that engages with its subject. Siddhartha Mukherjee's history of the gene and Evelyn Fox Keller's study of the concept are among the works on our reading lists, and building this depth is what our science tutoring and admissions support are for.
