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Chihuahuas remained a rarity until the early 20th century, and the American Kennel Club did not register a Chihuahua until 1904. In a 1520 letter, Hernan Cortés wrote that the Aztecs raised and sold the little dogs as food. Colonial records refer to small, nearly hairless dogs at the beginning of the 19th century, one of which claims 16th-century Conquistadores found them plentiful in the region later known as Chihuahua.
An analysis of DNA from the genome of domesticated dogs indicates that they entered North America from Siberia for 4,500 years and were then isolated for the next 9,000 years. After contact with Europeans, these lineages were replaced by Eurasian dogs and their local descendants. The pre-contact dogs exhibited a unique genetic signature that is now almost gone. In 2020, the sequencing of ancient dog genomes indicates that in two Mexican breeds the Chihuahua retains 4% and the Xoloitzcuintli 3% pre-colonial ancestry.
Chihuahuas are the smallest breed recognized by some kennel clubs.
Current breed standards defined by registries specify an "apple-head" or "apple-dome" skull conformation. Chihuahuas have large, round eyes and large, erect ears, set in a high, dramatically rounded skull. The stop is well defined, forming a near 90 degree angle where the muzzle meets the skull. Dogs of the older "deer" type, with a flat-topped head, more widely set eyes, larger ears, and longer, more slender legs, may still be registered.
Breed standards for this dog do not generally specify a height; only a weight and a description of their overall proportions. Generally, the height ranges between 6 and 9 in (15 and 23 cm); however, some dogs grow as tall as 30 to 38 cm (12 to 15 in). Both British and American breed standards state that a Chihuahua must not weigh more than 5.9 lb (2.7 kg) for conformation.
However, the British standard also states that a weight of 4–6 lb (1.8–2.7 kg) is preferred. A clause stating, "if two dogs are equally good in type, the more diminutive one is preferred" was removed in 2009. The Fédération Cynologique Internationale standard calls for dogs ideally between 1.5 and 3.0 kg (3.3 and 6.6 lbs), although smaller ones are acceptable in the show ring.
Pet Chihuahuas (those bred or purchased as companions rather than show dogs) often range above these weights, even above 10 lbs, if they have large bone structures or are allowed to become overweight. This does not mean that they are not purebred Chihuahuas; they just do not meet the requirements to enter a conformation show. Oversized Chihuahuas are seen in some of the best, and worst, bloodlines. Chihuahuas do not breed true for size, and puppies from the same litter can mature in drastically different sizes from one another. Also, larger breeding females are less likely to experience dystocia (obstructed labour). Many breeders try to breed Chihuahuas as small as possible, because those marketed as "teacup" or "tiny teacup" demand higher prices.
Chihuahuas occur in virtually any color combination, from solid to marked or splashed.
The Fédération Cynologique Internationale, which represents the major kennel clubs of 84 countries, disqualified the merle coat pattern, which appears mottled. In May 2007, The Kennel Club decided not to register puppies with this coloration due to the health risks associated with the responsible gene, and in December of that year, formally amended its breed standard to disqualify merle dogs.
The Chihuahua has some predisposition to several neurological diseases, among them atlantoaxial instability, ceroid lipofuscinosis, congenital deafness, congenital hydrocephalus, muscular dystrophy, necrotizing meningoencephalitis and neuroaxonal dystrophy.: 3  In a radiographical study of canine periodontal disease in 2001, the Chihuahua was found to have the lowest incidence of any of the six breeds studied.: 206 : 532
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- Media related to Chihuahua (dog) at Wikimedia Commons
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