The domestic dog is the first species, and the only large carnivore, known to have been domesticated. Especially over the past 200 years, dogs have undergone rapid phenotypic change and were formed into today's modern dog breeds due to artificial selection by humans. These breeds can vary in size and weight from a 0.46 kg (1.0 lb) teacup poodle to a 90 kg (200 lb) giant mastiff. Phenotypic variation can include height measured to the withers ranging from 15.2 centimetres (6.0 in) in the Chihuahua to 76 cm (30 in) 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.[131] The skull, body, and limb proportions vary significantly between breeds, with dogs displaying more phenotypic diversity than can be found within the entire order of carnivores. Some breeds demonstrate outstanding skills in herding, retrieving, scent detection, and guarding, which demonstrates the functional and behavioral diversity of dogs. The first dogs were domesticated from shared ancestors of modern wolves, however the phenotypic changes that coincided with the dog–wolf genetic divergence are not known.[26]
In 1999, a study of mitochondrial DNA indicated that the domestic dog may have originated from multiple grey wolf populations, with the dingo and New Guinea singing dog "breeds" having developed at a time when human populations were more isolated from each other.[23] In the third edition of Mammal Species of the World published in 2005, the mammalogist W. Christopher Wozencraft listed under the wolf Canis lupus its wild subspecies, and proposed two additional subspecies: "familiaris Linneaus, 1758 [domestic dog]" and "dingo Meyer, 1793 [domestic dog]". Wozencraft included hallstromi – the New Guinea singing dog – as a taxonomic synonym for the dingo. Wozencraft referred to the mDNA study as one of the guides in forming his decision.[1] The inclusion of familiaris and dingo under a "domestic dog" clade has been noted by other mammalogists.[24] This classification by Wozencraft is debated among zoologists.[25]
Clip the dog's hair methodically.[9] You can place the blade against the body safely, so long as you don't press the blade strongly into the skin. Brush against the direction of hair growth(back-brush) before running the clippers the other direction; with the direction of hair growth. Using the clippers against the growth of hair will have the same effect as back-brushing, but will leave a shorter length than the clipper blade that you are using. If you want to shave against the growth of hair, a blade size gets two blade sizes smaller when cutting against the grain. For example, when cutting with 4# against the grain check how long a 7# leaves behind when cutting with the grain. Move the clippers surely, but slowly across the dog's body to remove the hair — moving too fast might leave uneven lines. Always move the blade with the direction of the hairs' growth unless you want a shorter length than your blade claims to leave. Begin at the neck, then move down to the shoulders, under the ears, and toward the chin, throat, and chest areas. DO NOT use a size seven or any skip blades around the throat area or any flaps on the body that can fit between the teeth, such as the Achilles' tendons, arm pits, genital area, tip of tail, or anus. Then, clip the dog's back and sides. Finally, clip the hair on the dog's legs. Be careful when cutting the neck with any blade as it's the most dangerous to cut. Never cut straight down, do so at angles to avoid flaps you can't see in the neck getting cut.

There are many different shapes for dog tails: straight, straight up, sickle, curled, or cork-screw. As with many canids, one of the primary functions of a dog's tail is to communicate their emotional state, which can be important in getting along with others. In some hunting dogs, however, the tail is traditionally docked to avoid injuries.[38] In some breeds, such as the Braque du Bourbonnais, puppies can be born with a short tail or no tail at all.[39]
The role of the dog in Chinese mythology includes a position as one of the twelve animals which cyclically represent years (the zodiacal dog). Three of the 88 constellations in western astronomy also represent dogs: Canis Major (the Great Dog, whose brightest star, Sirius, is also called the Dog Star), Canis Minor (the Little Dog), and Canes Venatici (the Hunting Dogs).
Toxocara canis (dog roundworm) eggs in dog feces can cause toxocariasis. In the United States, about 10,000 cases of Toxocara infection are reported in humans each year, and almost 14% of the U.S. population is infected.[185] In Great Britain, 24% of soil samples taken from public parks contained T. canis eggs.[186][failed verification] Untreated toxocariasis can cause retinal damage and decreased vision.[186] Dog feces can also contain hookworms that cause cutaneous larva migrans in humans.[187][188][189][190]
There are many different shapes for dog tails: straight, straight up, sickle, curled, or cork-screw. As with many canids, one of the primary functions of a dog's tail is to communicate their emotional state, which can be important in getting along with others. In some hunting dogs, however, the tail is traditionally docked to avoid injuries.[38] In some breeds, such as the Braque du Bourbonnais, puppies can be born with a short tail or no tail at all.[39]
Be patient. You may have to go over your dog's coat a few times before you have a smooth, even cut. Don't rush it! Give your dog as many breaks as necessary, and make sure to move your clippers slowly. Brushing up against the coat then going the spot with clippers again help to get a smooth cut. Do will have to be done many times before you are done.
Run shower with lukewarm water. Hot water can harm your dog's skin, especially if your dog is short haired. Avoid running water right away on the dog, as it may cause unnecessary stress and burns if starts out hot without checking first. You may have to spend some time desensitizing your dog to the sound of running water with the help of his favourite treats. Always go slowly to avoid overwhelming your pet and making things worse for both of you. If you only have a tub and no shower, use a hose outside as filling a tub with water or only have a cup to run over the dog with water will not penetrate the fur or completely rinse off shampoo properly which causes infection as well as dry skin if completely soaked in water. This is true for even short hair dogs like pit bulls, you don't need the high power setting of any hose only enough to not hurt or stress the dog but still gets the job done.
In a study of seven breeds of dogs (Bernese mountain dog, basset hound, Cairn terrier, Epagneul Breton, German Shepherd dog, Leonberger, and West Highland white terrier) it was found that inbreeding decreases litter size and survival.[84] Another analysis of data on 42,855 dachshund litters found that as the inbreeding coefficient increased, litter size decreased and the percentage of stillborn puppies increased, thus indicating inbreeding depression.[85] In a study of boxer litters, 22% of puppies died before reaching 7 weeks of age.[86] Stillbirth was the most frequent cause of death, followed by infection. Mortality due to infection increased significantly with increases in inbreeding.[86]

Dogs are highly variable in height and weight. The smallest known adult dog was a Yorkshire Terrier, that stood only 6.3 cm (2.5 in) at the shoulder, 9.5 cm (3.7 in) in length along the head-and-body, and weighed only 113 grams (4.0 oz). The largest known dog was an English Mastiff which weighed 155.6 kg (343 lb) and was 250 cm (98 in) from the snout to the tail.[31] The tallest dog is a Great Dane that stands 106.7 cm (42.0 in) at the shoulder.[32]
Some breeds of dogs are prone to certain genetic ailments such as elbow and hip dysplasia, blindness, deafness, pulmonic stenosis, cleft palate, and trick knees. Two serious medical conditions particularly affecting dogs are pyometra, affecting unspayed females of all types and ages, and gastric dilatation volvulus (bloat), which affects the larger breeds or deep-chested dogs. Both of these are acute conditions, and can kill rapidly. Dogs are also susceptible to parasites such as fleas, ticks, mites, hookworms, tapeworms, roundworms, and heartworms (roundworm species that lives in the heart of dogs).
Take extra care with dark-nailed dogs not to clip to the quick (blood vessel). Go slowly, and only take a little bit off at a time. Dremeling is much safer and easier to avoid hitting the quick, as it shaves only a little bit at a time. Use a cordless pet-safe Dremel tool, as the corded ones will not stop turning if they catch hair. Don't dremel for too long as it will burn the nail and prolong the trimming process for the dog which causes more stress. The best recommended process is trim first then dremel to shorten a little bit and round out the nail do it's not sharp.
Comb out your dog first.[1] Combing your dog's coat daily or every other day will keep most mats at bay. Simply brushing, as most literature instructs, is not enough for dogs that can mat up: the brush will easily pass over at angles that a comb will get stuck on. A thorough combing should always be the first step of the grooming process because any mats will become tighter and less manageable once they dry. Begin on the head and move down the body. Be careful under the belly, as it is a sensitive area, and don't forget to comb the tail.