Most breeders of purebreds used eugenics in their standard, as set by The Kennel Club. Before their were breed standards to follow, breeders were free to create their own lines freely, with a outcrossing or two to sure up their lines. This also created a more vigorous and healthy dog. To understand this you must understand dog breeding. A good book to read that all breeders should read is Breeding Better Dogs by Carmelo L. Battaglia. Health issues often seen in the Yorkshire Terrier such as
bronchitis,
lymphangiectasia,
portosystemic shunt,
cataracts, and
keratitis sicca,
Luxating patellas,
distichiasis,
hypoplasia of dens,
Legg–Calvé–Perthes syndrome,
hydrocephalus,
retinal dysplasia,
tracheal collapse, and
bladder stones can be bred out if breeders come together for the health betterment of the breed. When it comes to breeding better dogs test can tell us a lot, along with time and a well trained eye on your side is the road to success.