The human genome contains more than 20,000 protein-coding genes, which carry the instructions for proteins essential to the structure and function of our cells, tissues and organs. Some of these genes are very similar to each other because, as the genomes of humans and other mammals evolve, glitches in DNA replication sometimes result in extra copies of a gene being made. Those duplicates can be passed along to subsequent generations and, on very rare occasions, usually at a much later point in time, acquire additional modifications that may enable them to serve new biological functions. By starting with a protein shape that has already been fine-tuned for one function, evolution can produce a new function more rapidly than starting from scratch.
Pretty cool! But it leads to a question that’s long perplexed evolutionary biologists: Why don’t duplicate genes vanish from the gene pool almost as soon as they appear? After all, instantly doubling the amount of protein produced in an organism is usually a recipe for disaster—just think what might happen to a human baby born with twice as much insulin or clotting factor as normal. At the very least, duplicate genes should be unnecessary and therefore vulnerable to being degraded into functionless pseudogenes as new mutations arise over time
An NIH-supported team offers a possible answer to this question in a study published in the journal Science. Based on their analysis of duplicate gene pairs in the human and mouse genomes, the researchers suggest that extra genes persist in the genome because of rapid changes in gene activity. Instead of the original gene producing 100 percent of a protein in the body, the gene duo quickly divvies up the job . For instance, the original gene might produce roughly 50 percent and its duplicate the other 50 percent. Most importantly, organisms find the right balance and the duplicate genes can easily survive to be passed along to their offspring, providing fodder for continued evolution.