Some breeders will try to tell you that they repeat breedings and get identical litters. This is a FALLACY. The litters may look alike - or not - but their genetic make-up can never be identical.
The most simple and best article written on breeding and genetics was reprinted in the Dog World, January, 1973.The article was written by Dr.Lee Ford and was originally published in England's "Dog World" in April 2nd, 1971, under the title "Genetic Basis of Variations."
Dr.Ford explains clearly why variations must be understood in order to profit by the study of genetics in breeding. Variations explain why you can have a great repeat breeding but NEVER an identical breeding. One of the best paragraphs in Dr.Ford's article is the finale one. It is headed "Genetic Drift": - Genetic Drift is CHANCE. One should understand the importance of chance in ALL dog breeding, selective matings or not, as a chance re-combination of genes occurs despite the best efforts of the dog breeder.
Without the knowledge that chance is also operating, one could not possibly hope to make progress in dog breeding of any kind. Therefore, the best the dog breeder can do to improve his line or breed, is to take all factors involved in heredity into consideration, all factors in environment into consideration, weigh them one against the other, and hope that chance (luck) is with them. Because, in spite of his effort and work, chance or genetic drift could just wreck havoc with all his efforts.
The more that I breed (says the writer A.Martello) the older I get and the more I study, the more I realize how important that paragraph is. If I had realized this years ago, it would have saved me a lot of heartaches. Owning dogs, loving them and being loved by them; mating them and watching the miracle of birth; watching them develop; is a rare privilege. We learn many things, both good and bad, from being able to do these things, and apparently, we are allowed to learn just so much. Oh well, how could we really enjoy our dogs if there had ever been a "perfect one."