Many breeders plan matings solely on the appearance (phenotype) of a dog and not on its pedigree or the relatedness of the prospective parents. Matings based on appearance are called "assortative matings". There are positive assortative matings (like-to-like) and negative assortative matings (like-to-unlike) for individual traits. Breeders use positive assortative matings when they wish to fix traits, and negative assortative matings when they wish to correct traits or bring in traits the breeding stock may lack.
Some individuals may share desirable characteristics, but they inherit them differently. This is especially true of polygenetic traits, such as ear set, bite or length of forearm. Breeding two phenotypically similar but geno-typically unrelated dogs together would not necessarily reproduce these traits.
Conversely, each individual with the same pedigree will not necessarily look or breed alike.
Therefore, breedings should not be planned solely on the basis of the pedigree or appearance alone. Matings should be based on a combination of appearance and ancestry. If you are trying to fix a certain trait - like topline - and it is one you can observe in the parents and the linebred ancestors of two related dogs, then you can be more confident that you will attain your goal.
If a linebreeding produces a puppy with magnificent qualities, but those qualities are not present in any of the ancestors the pup has been linebred on, then the dog may have a wonderful show career, but it may not breed true. Therefore, careful selection of mates is important, but careful selection of puppies from the resultant litter is also important to fulfill your genetic goals.

Copyright (R) 2000 National New and Rare Breed Association