Normally, it takes years for genetic changes to spread through a population. That is because, during sexual reproduction, each of the two versions of any gene has only a fifty per cent chance of being inherited. But a gene drive which is named for its ability to propel genes through populations over many generations manages to override the traditional rules of genetics. A mutation made by CRISPR on one chromosome can copy itself in every generation, so that nearly all descendants would inherit the change. A mutation engineered into a mosquito that would block the parasite responsible for malaria, for instance, could be driven through a large population of mosquitoes within a year or two. If the mutation reduced the number of eggs produced by that mosquito, the population could be wiped out, along with any malaria parasites it carried.
Kevin Esvelt, an evolutionary biologist at Harvard, was the first to demonstrate how gene drives and CRISPR could combine to alter the traits of wild populations. Recently, he has begun to study the possibility of using the technology to eliminate Lyme disease by rewriting the genes of mice in the wild. Lyme disease is caused by a bacterium and transmitted by ticks, and more than eighty-five per cent of the time they become infected after biting a mouse. Once exposed, however, some mice naturally acquire resistance or immunity.
"My idea is to take the existing genes that confer resistance to Lyme and make sure that all mice have the most effective version", Esvelt said. To do that, scientists could encode the most protective genes next to the CRISPR system and force them to be passed on together. Esvelt stressed that such an approach would become possible only after much more research and a lengthy series of public discussions on the risks and benefits of the process.
Last year, by deleting all three copies of a single wheat gene, a team led by the Chinese geneticist Gao Caixia created a strain that is fully resistant to powdery mildew, one of the world's most pervasive blights. In September, Japanese scientists used the technique to prolong the life of tomatoes by turning off genes that control how quickly they ripen. Agricultural researchers hope that such an approach to enhancing crops will prove far less controversial than using genetically modified organisms, a process that requires technicians to introduce foreign DNA into the genes of many of the foods we eat.
The technology has also made it possible to study complicated illnesses in an entirely new way. A few well-known disorders, such as Huntington's disease and sickle-cell anemia, are caused by defects in a single gene. But most devastating illnesses, among them diabetes, autism, Alzheimer's, and cancer, are almost always the result of a constantly shifting dynamic that can include hundreds of genes. The best way to understand those connections has been to test them in animal models, a process of trial and error that can take years. CRISPR promises to make that process easier, more accurate, and exponentially faster.