The small white-fox working terriers we know today were first bred by the Reverend John Russell, a parson and hunting enthusiast born in 1795, and they can trace their origin to the now extinct English White terrier.
Difficulty in differentiating the dog from the creature it was pursuing brought about the need for a mostly white dog,and so in 1819 during his last year of university at Exeter College, Oxford, he purchased a small white and tan terrier female named Trump from a milkman in the nearby small hamlet of Elsfield.
Trump epitomized his ideal Fox terrier, which, at the time, was a term used for any terrier which was used to bolt foxes out of their burrows. Her colouring was described as "...white, with just a patch of dark tan over each eye and ear; whilst a similar dot, not larger than a penny piece, marks the root of the tail." Davies, a friend of Russell's, wrote "Trump was such an animal as Russell had only seen in his dreams". She was the basis for a breeding program to develop a terrier with high stamina for the hunt as well as the courage and formation to chase out foxes that had gone to ground. By the 1850s, these dogs were recognised as a distinct breed
Jack Russell terriers come in a variety of coat types, and with a range of markings.
Due to their working nature, Jack Russell terriers remain much as they were some 200 years ago. They are sturdy, tough, and tenacious, measuring between 10–15 inches (25–38 cm) at the withers, and weigh 14–18 pounds (6.4–8.2 kg).
The body length must be in proportion to the height, and the dog should present a compact, balanced image. Predominantly white in coloration (more than 51%) with black and/or tan markings, they exhibit either a smooth, rough or a combination of both which is known as a broken coat. A broken-coated dog may have longer hair on the tail or face than that which is seen on a smooth-coated dog. The skin can sometimes show a pattern of small black or brown spots, referred to as "ticking" that do not carry through to the outer coat. All coat types should be dense double coats that are neither silky (in the case of smooth coats) nor woolly (in the case of rough coats)
Jack Russells are first and foremost a working terrier. Originally bred to bolt fox from their dens during hunts, they are used on numerous ground-dwelling quarry such as groundhog, badger, and red and grey fox. The working JRT is required to locate quarry in the earth, and then either bolt it or hold it in place until they are dug to. To accomplish this, the dog won't bark but will expect attention to the quarry continuously. Because the preservation of this working ability is of highest importance to most registered JRTCA/JRTCGB breeders, Jack Russells tend to be extremely intelligent, athletic, fearless, and vocal dogs. It is not uncommon for these dogs to become moody or destructive if not properly stimulated and exercised, as they have a tendency to bore easily and will often create their own fun when left alone to entertain themselves.
The breed has a reputation for being healthy with a long lifespan. Breeders have protected the gene pool, and direct in-line breeding has been prevented. Jack Russells can live anywhere from between 13 to 16 years on average given proper care. However certain lines have been noted for having specific health concerns, and therefore could occur in any line or generation because of recessive genes. These issues can include hereditary cataracts, ectopia lentis, congenital deafness, patellar luxation, ataxia, myasthenia gravis, Legg–Calvé–Perthes syndrome and von Willebrand disease.