According to the first National Mutt Census results just released, the German Shepherd Dog (GSD) is the most common breed found in America’s mutts.
Per Veterinary Practice News:
While German Shepherds prove popular as both a pure breed and mixed breed, the most common breeds registered by the American Kennel Club are not necessarily the most common breeds found in mixed breed dogs, according to Mars Veterinary.
A little over half of all pet dogs in the U.S. are of blended heritage. According to the census, 46 percent of mixed breed dogs are adopted from shelters, and another 18 percent are acquired from friends, neighbors and relatives.