Crash Course in Genetics: The crisis in defining genes

Last week, we explored what a gene is (please check it out if you haven’t, it’ll help this make sense) and mentioned that scientists actually aren’t quite so sure about what a gene is anymore.

 

The HGP and genes today

The Human Genome Project (HGP) aimed to sequence the entire human genome to build a far greater understanding of all the human genes to aid in medical and genetic research and building of knowledge. The project itself was a success, but it uncovered a number of unexpected results as well. For starters, scientists had estimated that humans had over 100,000 different genes in order to account for our complexity. Instead, humans have only about 20,000, which was an astonishing result given that many flowering plants have more than 40,000 genes! It became very clear that number of genes was not an effective indicator of complexity and called into question the concept of individual genes coding for individual proteins or functions.

 

So what is a gene?

More recent research shows that non-protein-coding DNA doesn’t fit into the defined boundaries of a gene and that there are at least twice as many non-coding DNA segments  and possibly 600,000 or even 3 million of these small regulatory sections that aren’t traditional genes. This means that we can’t just list all genes and their functions, as the HGP had originally aimed to do and our understanding of genetics has become far more complicated. Rather than a list of instructions, we’re faced with an interconnected web that is interspersed with protein-coding segments, regulatory snippets of DNA and some other stuff we’re not quite sure about yet. Some sections will change other sections and impact them even when they are spatially very far apart, other seperate pairs will work together to get a ‘fused’ result, and far more.

Scientists are realising that the word gene, and its definition as a “unit of heredity” is outdated and doesn’t address the inherent complexity of modern genetics. Other groups are beginning to refer to molecular processes to describe a function of DNA in a cell. Others have proposed changing the definition altogether:

A gene is a DNA sequence (whose component segments do not necessarily need to be physically contiguous) that specifies one or more sequence-related RNAs/proteins that are both evoked by GRNs and participate as elements in GRNs, often with indirect effects, or as outputs of GRNs, the latter yielding more direct phenotypic effects.”

In more normal wording, this means that a gene is a section of DNA or sections that are split up. Each of these sections codes for an RNA chain that can transfer the information or a protein. These sections are controlled by and can control other genetic regulatory networks (GRNs), a term that encompasses processes that affect when and which genes are expressed and how they are (in other words, genes can turn other genes on and off and affect how they function). When genes do this, they can affect an individual’s phenotype, the combination of observed factors, such as height, skin colour, other features, etc…

 

This redefinition encompasses the different types and modes of function that scientists have identified in recent science which makes it a very usable definition. It is likely that something of this form will eventually become a mainstream definition included in textbooks and papers. Hopefully, this complexity will also be communicated to the general public to help prevent the pervasive beliefs that single sections of DNA cause illnesses and build physical features, when in fact it is a combination of many factors and interactions to produce an overall result.


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