WHETHER they are humans or pea plants, the way living organisms look and behave is intimately connected with their genes. But ideas of genes and their workings have in the century since the word was coined.
In essence it is simple. 鈥淎 gene is the stretch of DNA letters that encodes individual functional units or proteins,鈥� says of the Broad Institute in Cambridge, Massachusetts. Within our cells, double-stranded DNA is continually unzipped and transcribed into single-stranded RNA, which performs cell functions itself or can be used as a template to assemble the proteins that make us what we are.
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As such, genes are basic, universal units of heredity. You generally have two copies of each, one from your mother and one from your father. Each gene has different versions that vary slightly at the molecular level, generating different outward effects 鈥� brown or blue eyes, for example.
We long thought there must be one gene for each outward characteristic, but that belief hasn鈥檛 stood the test of time. The Human Genome Project, completed in 2003, revealed that genes account for just 1 per cent of our DNA; the rest is 鈥渏unk鈥�. We have only 20,000 genes in total 鈥� far too few for the one gene, one function idea.
It turns out that genes do different things depending on factors like when and where they are expressed. 鈥淕enes can be alternatively spliced, which means that different chunks of them can be encoded into a protein,鈥� says geneticist at the University of Exeter, UK. In other words, rather than a gene coding for only one protein, different bits of the genetic code can get chopped out to create different proteins. All this is regulated by non-gene DNA, RNA molecules and other proteins, further diluting a gene鈥檚 autonomy in determining a protein.
This means it can be very hard to pin down which genes are responsible for any given thing. 鈥淚t鈥檚 very rare that there鈥檚 a gene for something,鈥� says Gabriel 鈥� even a trait as seemingly simple as eye colour.
Other discoveries have changed how we think about things, too. A single gene is comprised of segments of DNA, but one segment can contribute to more than one gene, meaning that genes can have overlapping boundaries. In some organisms, , scattered around the genome. In addition, not all genes code for proteins 鈥� many simply code for a variety of RNA molecules.
To cap all that there is the emerging science of epigenetics: the realisation that genes can be switched on and off in response to environmental factors such as stress or diet, meaning that the effects of some genes disappear in some generations only to reappear in subsequent ones.
Given all this, is it even useful to think of genes at all? 鈥淒efinitely,鈥� says Gabriel, given all they do. But some people believe we need to redefine them. Petter Portin, a geneticist at the University of Turku in Finland, argues that thinking of a gene as a universal unit of heredity is too simple. 鈥淚n many situations it would be more helpful to think about genes as small cogs in a much larger machine,鈥� he says.
This article appeared in print under the headline 鈥淗ow to think about鈥� Genes鈥�
