Scientists have analysed thousands of modern dog genomes and found that up to 64% of dog breeds carry traces of wolves in their DNA, the result of interbreeding over thousands of years. The highest proportion of wolf DNA has been identified in breeds close to their wild ancestors, for example the Czechoslovakian Wolfhound has 11-12% wolf DNA and the Saarloos Wolfhound has 18-33% wolf DNA. Interestingly, wolf DNA is also found in small breeds such as the Chihuahua, which has approximately 0.2% wolf genetic makeup. This ancient genetic influence affects the physical characteristics of dogs, such as body size, and also their behaviour or resistance to the environment. Research also shows that the origin of the dogs goes back to at least two separate wolf populations that mixed during domestication. These findings confirm the deep genetic link between dogs and wolves that persists in today's domesticated breeds.