here's a start in in line breeding and breeding sisters to brothers. i hope tim doesn't get mad that i posted this but i'm sure he'd rather all of us get educated. Lets say a person crosses rooster A with hen B. An organism can only give 50% of its genes to its offspring. The F1 offspring will have 50% of roosters A's genes and 50% of hen B's genes. So the F1 are 50A/50B. If a person crosses two F1 50A/50B x 50A/50B (producing F2), then the F1 rooster can only give 25% of the A genes to the F2 and the hen will provide 25% of the A genes also, equalling 50% A genes in the F2 the same is true for the B genes. The F2 offspring are 50A/50B just like the F1 parents. If a person back crosses the F1 50A/50B to Rooster A (100% A), then the formula changes and the offspring are more like the rooster genetically- 75%A/25%B. If a person back crosses to the hen- then the offspring are more like her 25%A/75%B. The problem arises when there are unwanted genes that are recessive in a flock. If a person crosses two birds that are carriers of a recessive gene( they do not express the trait), they can have chicks that do not have the trait and chicks that do have the trait. You cull the birds that have the trait. Some of the birds that do not have the trait will be carriers so they can pass the gene on to their offspring. Some of the offspring will not even carry the gene so they can not even give the gene to their offspring. You can eliminate the trait from your flock by crossing siblings or you can encourage the trait by crossing siblings but you cull the bad birds. Culling is the key to inbreeding. Once a breeder has a flock that is genetically homogeneous then a breeder must out cross to improve traits. Lets say a person is wanting to improve a specific trait, and the breeder crosses a specific male to a specific female and hatches 30 eggs from the crossing and none of the chicks have improved on the trait- then genetically there is nothing to be gained by crossing the two again. A breeder will have to out cross to get the gene or genes needed to improve the trait.
I had a professor in my veterinary classes in college who said, "It's linebreeding when it works & inbreeding when it doesn't."