In any breed, variety, or subvariety certain families are sometimes distinguished for general or special excellence of form or color. Such a family is called a strain.
Systematic mixtures of breeds and varieties. Although so many distinct varieties have been developed from common domestic stocks, the improved races do not always displace the mongrels. When the old mongrels disappear their place is often taken by a new mongrel stock produced by mixtures of the distinct breeds with each other and with the old mongrel race. The greater part of such stock is so mixed that its relation to any established breed could not be determined or expressed, but systematic mixtures are sometimes made, and to describe these the following terms are used: Crossbred—having parents of different, distinct breeds, varieties, or subvarieties. A Leghorn male mated with a Cochin female produces offspring each of which is in blood one half Leghorn and one half Cochin. Grade—having more than half of the blood of a breed.
If the offspring of a cross such as is described in the preceding paragraph are mated with birds of one of the parent breeds, the offspring of this mating will have three fourths of the blood of that breed. If these in turn are mated to birds of the same pure breed, the offspring will have seven eighths of the blood of that breed. Animals bred in this way are called grades until the process has been carried so far that they are practically pure-bred. Mongrel stock is often graded up in this way. As a rule stock that is seven eighths pure is not distinguishable from average pure stock of the same breed.
Pure-bred, thoroughbred, and standard-bred. A pure-bred animal is, strictly speaking, one having the blood only of the variety to which it belongs. From what has been said of the making of breeds and varieties it is plain that absolute purity of blood is not a universal attribute of well-bred domestic birds. A thoroughbred animal is one that is thoroughly bred for some purpose or to some type. A standard-bred animal is one that is bred especially to conform to requirements agreed upon by breeders and exhibitors.
A great deal of misapprehension and confusion in the use of these terms has been caused by the attitude of those who maintain that the term "thoroughbred," having been used as a name for highly bred running horses, cannot properly apply to any other kind of live stock, and that "pure-bred" should apply to all thoroughly bred races. The noun "Thoroughbred" is the name of a breed of horses. The adjective "thoroughbred" is common property. Writers on aviculture who wish to be accurate prefer it in many instances to "pure-bred" because absolute purity of blood is rare and is not of the importance in breeding that novices usually suppose. Not only are many new varieties made by crossing, but in long-established breeds outcrosses are regularly made to restore or intensify characters.
To illustrate the use of the three terms in application to a single breed: A stock of Light Brahmas might be kept pure for half a century, yet at the end of that period might have changed its type entirely. It might be so deteriorated that it was worth less than common mongrels; yet it is pure-bred stock. Another stock of the same variety might be bred for table qualities, egg-production, and the same principal color-characteristics of the variety, but without attention to the fine points of fancy breeding. Such a stock is thoroughbred but not standard-bred.