The process of bringing in another breed into your breeding program is called grading. This is often done in cattle and other species. Here is a snippet on grading from an SPPA article.
Grading: (Craig Russell describes this much better than I can)
(from SPPA Breeding Method B – by Craig Russell
Taken from SPPA Bulletin, spring 1999, volume 4, #1)
Grading in breeding is the process by which a population can be “improved” in terms of show points, production, or both. A mixed population or even another breed or variety can be converted to a desired strain, variety, or breed. In practice as well as in theory you could start with a flock of White Crested Black Polish, Cornish Rock crosses, Light Brahmas, or anything else and in eight years (or eight breeding seasons that can be done in four or five years) have a “pure” population of, say, Red Dorkings.
Let’s return to our Red Dorking trio. At the time you purchased them you had eight Cornish Rock hens. These could be added to the original mating. The pullets from the Cornish Rock hens would be mated back to pure Dorking males the following year. The cockerels and the old Cornish Rock hens could be sold or eaten. Any of the pullets could be used in the breeding process, but normal would speed up the process. So if you are going to use ten pullets, pick those with ten toes and some red feathers. In future generations continue to select for birds closest to the desired color and type and cull those with undesirable traits.
The following chart shows the progression in fractions, decimals and percentages from year to year. This works for any breed, but in the example we are grading Cornish Rocks crossed to Red Dorkings.
Year Fraction Decimal Percent Breed
1 ½ .5 50 Red Dorking
2 3/4 .75 75 Red Dorking
3 7/8 .875 87.5 Red Dorking
4 15/16 .9375 93.75 Red Dorking
5 31/32 .96875 96.875 Red Dorking
6 63/64 .984375 98.4375 Red Dorking
7 127/128 .9921875 99.21875 Red Dorking
8 255/256 .99615375 99.615375 Red Dorking
This process can be continued as long as you like, but for all practical purposes eight cycles yields pure stock. Most large stock with open registries grants pure status after six generations. In cases where one variety is being graded to another or one strain of a variety or breed is being upgraded by the addition of another strain, far fewer cycles are usually required before all of the offspring can be returned to the regular mating system. This is an old system long used by professionals in cattle, horses, swine, sheep, goats, dogs and other stock. Javas and Jersey Blues became Jersey Giants nearly a century ago utilizing this method. In that case, as Giants became a hot item some breeders just started calling their Javas and black Jersey “Blues” Giants and selected breeders with a little more leg. You get the idea.
Developing a new variety or strain differs from grading in that usually half bloods or in some cases three quarter bloods are mated brother to sister and selected. Once birds start to show the desired traits, a combination of rolling matings and grading fix desirable traits and build up the population.
