The color conformation is quite varied, but solid black or any degree of merle is considered a cosmetic fault, and a blue color is a disqualification by the NKC Breed Standard. Centuries of selective breeding by humans has resulted in dogs being more genetically diverse than most other mammals by a considerable margin.
George Washington received French Foxhounds, Grand Bleu de Gascogne, (which look much like an American Bluetick Coonhound) as a gift from the Marquis de Lafayette. Eye color is usually brown, but heterochromia also occurs, although considered a cosmetic fault. For dogs where hairlessness is a dominant gene, hairless to hairless matings will on average produce 66.6% hairless and 33.3% coated puppies.
It is estimated that three-quarters of the world's dog population lives in the developing world as feral, village or community dogs, with pet dogs uncommon. "The most widespread form of interspecies bonding occurs between humans and dogs" and the keeping of dogs as companions, particularly by elites, has a long history. (As a possible example, at the Natufian culture site of Ain Mallaha in Israel, dated to 12,000 BC, the remains of an elderly human and a four-to-five-month-old puppy were found buried together). However, pet dog populations grew significantly after World War II as suburbanization increased. In the 1950s and 1960s, dogs were kept outside more often than they tend to be today (using the expression "in the doghouse" to describe exclusion from the group signifies the distance between the doghouse and the home) and were still primarily functional, acting as a guard, children's playmate, or walking companion. Truman. Young American Bulldogs may be slightly aloof with strangers, but as they mature the breed's normal confidence should assert itself. It is not uncommon for an American Bulldog to require a high level of attention due to their highly emotional personality.