Size is generally believed to be polygenic; it is a trait that is controlled by a number of genes with each gene having a small effect as opposed to being controlled by one gene. A smaller dog has the gene combination present that give it that small size, vice versa for the larger dog. Although littermates have genes in common, breeding to one dog from a litter is NOT the same as breeding to a littermate. The combination of their genes (genotype) differs. In a trait like size where there is a visible expression of the particular dog's genetic make-up (phenotype), you are better off using the littermate whose phenotype is what you want because you are more likely to have the correct genotype to pass on or help pass on the trait you desire. Not always, but your chances are better.
Remember the real question is pelvic span versus just general size in deciding whether a bitch can free whelp. You also need to take the genetics of whelping behaviors into account too (dystocia is believed to be a genetic trait).
You all are probably sick to death of the recommendation but Claudia Orlandi's ABCs of Dog Breeding is an excellent source for information concerning canine genetics. There are lots of misconceptions about how inheritance works and it does a great job of explaining the science of dog breeding. |