A hairless dog is a dog

Dog

The domestic dog is a domesticated form of the gray wolf, a member of the Canidae family of the order Carnivora. The term is used for both feral and pet varieties. The dog may have been the first animal to be domesticated, and has been the most widely kept working, hunting, and companion animal in...

 with a genetic

Genetics

Genetics , a discipline of biology, is the science of genes, heredity, and variation in living organisms....

 disposition for hairlessness. There are two known types of genetic hairlessness, a dominant and a recessive

Recessive

In genetics, the term "recessive gene" refers to an allele that causes a phenotype that is only seen in a homozygous genotype and never in a heterozygous genotype. Every person has two copies of every gene on autosomal chromosomes, one from mother and one from father...

 type.

Dominant genes

Dogs with dominant genes for hairlessness can pass their attributes to their offspring in natural conditions, that is, not under the control of humans. Therefore, it is possible that, in some parts of the world, groups of hairless dogs came into existence without human intervention

Selective breeding

Selective breeding is the process of breeding plants and animals for particular genetic traits. Typically, strains that are selectively bred are domesticated, and the breeding is sometimes done by a professional breeder. Bred animals are known as breeds, while bred plants are known as varieties,...

. Later in history, people developed these groups into a recognized breed.

Worldwide recognized breeds at this time are the Chinese Crested Dog

Chinese Crested Dog

The Chinese crested dog is a smaller hairless breed of dog. Like most hairless dog breeds, the Chinese crested comes in two varieties, both with and without fur, which are born in the same litter: the Hairless and the Powderpuff....

, the Mexican Hairless Dog

Mexican Hairless Dog

The Mexican Hairless Dog is a rare, hairless breed of dog, the size of which varies greatly. It is also known as Xoloitzcuintle .-History:...

, and the Peruvian Hairless Dog

Peruvian Hairless Dog

The Peruvian Hairless Dog is a breed of dog with its origins in Peruvian pre-Inca cultures. It is one of several breeds of hairless dog. It is not to be confused with the Xoloitzcuintli.- Appearance :...

.

Other breeds that were said to have existed in the past were the African Hairless Dog (also known as the Abyssinian Sand Terrier, Egyptian Hairless Dog and Elephant Dog, the last being a reference to its grey skin) and the Thai Hairless Dog.

This type of genetic structure is said to be homozygous pre-natal lethal for the dominant gene. This means that zygotes with two dominant genes cannot live. Therefore, all dominant-hairless dogs have a heterozygous gene structure. This allows a homozygous recessive type to persist, which is the coated variety.

For dogs where hairlessness is a dominant gene, hairless to hairless matings will on average produce 66.6% hairless and 33.3% coated puppies. For hairless to coated matings, there will be an average of 50%/50% coated to hairless ratio, while for coated to coated matings, all puppies will be coated

The Chinese Crested coated variety is called "Powder Puff", and is a recognized type. The coated variety of Xoloitzcuintli (Mexican Hairless Dog) is recognized as a valid type for conformation showing with some registries, including the AKC. Coated varieties of the other breeds may or may not be recognized as valid varieties for conformation show

Conformation show

Conformation shows, also referred to as breed shows, are a kind of dog show in which a judge familiar with a specific dog breed evaluates individual purebred dogs for how well the dogs conform to the established breed type for their breed, as described in a breed's individual breed standard.A...

ing, depending on the breed and show or registry organization.

Recessive Genes

The American Hairless Terrier

American Hairless Terrier

The American Hairless Terrier is a breed of dog that was formerly considered a variant of Rat Terrier. As of January 1, 2004, the United Kennel Club deemed the AHT a separate terrier breed...

 (AHT) is the only dog breed with a recessive gene. Unlike the hairlessness resulting from dominant genes, the AHT has no dental, skin or other health issues associated with the dominant gene hairless breeds. The only AHT distinguishing characteristics from its coated Rat Terrier

Rat Terrier

The Rat Terrier is an American dog breed with a rich and varied background as an all-around farm dog and hunting companion. Traditionally more of a type than a breed, they share much ancestry with the tough little mixed-breed dogs known as feists...

foundation stock is its hairlessness. Other distinguishing features, such as colors and size, have been breeder induced.