Many years ago when I was studying genetics the question was raised about the difference between "line-breeding" and "in-breeding". The professor said the main difference is if you are doing it, it is "line-breeding", if your neighbor is doing it, it is "in-breeding". Another definition is if it is working then it is "line-breeding" but if it is making a mess of things then it is "in-breeding". Line-breeding is in-breeding and has the same results of fixing genes, both good and bad. When you mate relatives you will strengthen any good traits by concentrating the genes for that trait but by the same token you will expose any bad traits in your stock. By using in breeding techniques you can create lines of animals/birds which when cross with another line is called "out-breeding" and does have some heterosis effect.
I wouldn't disagree with this - except for the part about whether you are doing it or a neighbor is. What is, is what is. I would say though that "in breeding" as defined by Brother x Sister matings are more likely to concentrate the genes and expose the traits (good or bad) much faster than father to daughter and so on down the line. One thing I didn't mention before that I remembered and will here is the notion of using a rooster twice and then using one of his offspring. I used to hold to that too but a recent article by Kenny Troiano in the PP has convinced me that is a mistake. I'll be using my roosters from here on out for as long as 5-6 years unless I get something that is significantly better before then. Since I still don't have a foundation flock yet, that is possible but my point is that I won't be so quick to change out roosters. Nor will I be breeding pullets as much as hens from now on either. He addresses that as well.
God Bless,