Genetically modified squirrels could curb growing population of greys Researchers say releasing 100 mutated squirrels into a population of 3,000 greys would wipe out the population within 15 years Scientists suggest genetically altering squirrels to pass on infertility genes Credit: Peter Byrne/PA Mutant grey squirrels, genetically modified to spread infertility genes, could be released into the wild to tackle the burgeoning population, the University of Edinburgh has said. North American grey squirrels were imported to Britain in the mid-19th century by landowners, and their population has now grown to more than two million. Not only do they out-compete the native red squirrel, they also strip trees of their bark, causing a threat to woodlands, as well as preying on eggs and chicks.