Despite their close genetic relationship and the ability to inter-breed, there are a number of diagnostic features to distinguish the gray wolves from domestic dogs. Domesticated dogs are clearly distinguishable from wolves by starch gel electrophoresis of red blood cell acid phosphatase. The tympanic bullae are large, convex and almost spherical in gray wolves, while the bullae of dogs are smaller, compressed and slightly crumpled. Compared with equally sized wolves, dogs tend to have 20% smaller skulls and 30% smaller brains.:35 The teeth of gray wolves are also proportionately larger than those of dogs. Dogs have a more domed forehead and a distinctive "stop" between forehead and nose. The temporalis muscle that closes the jaws is more robust in wolves.:p158 Wolves do not have dewclaws on their back legs, unless there has been admixture with dogs that had them. Most dogs lack a functioning pre-caudal gland and enter estrus twice yearly, unlike gray wolves which only do so once a year. So-called primitive dogs such as Dingoes and Basenjis retain the yearly estrus cycle. Dogs generally have brown eyes and wolves almost always have amber or light colored eyes. The skin of domestic dogs tends to be thicker than that of wolves, with some Inuit tribes favoring the former for use as clothing due to its greater resistance to wear and tear in harsh weather. The paws of a dog are half the size of those of a wolf, and their tails tend to curl upwards, another trait not found in wolves The dog has developed into hundreds of varied breeds, and shows more behavioral and morphological variation than any other land mammal. For example, height measured to the withers ranges from a 6 inches (150 mm) in the Chihuahua to 3.3 feet (1.0 m) in the Irish Wolfhound; color varies from white through grays (usually called "blue") to black, and browns from light (tan) to dark ("red" or "chocolate") in a wide variation of patterns; coats can be short or long, coarse-haired to wool-like, straight, curly, or smooth. It is common for most breeds to shed their coat.
Unlike other domestic species which were primarily selected for production-related traits, dogs were initially selected for their behaviors. In 2016, a study found that there were only 11 fixed genes that showed variation between wolves and dogs. These gene variations were unlikely to have been the result of natural evolution, and indicate selection on both morphology and behavior during dog domestication. These genes have been shown to affect the catecholamine synthesis pathway, with the majority of the genes affecting the fight-or-flight response (i.e. selection for tameness), and emotional processing. Dogs generally show reduced fear and aggression compared with wolves. Some of these genes have been associated with aggression in some dog breeds, indicating their importance in both the initial domestication and then later in breed formation.