Did you know that a litter of puppies can have more than 1 sire (father)? That is one reason why not all puppies from a litter look alike. Sometimes genetics is the sole determining factor in what we see. On the other hand, male sperm can live up to a week in the female dog's reproductive system. When the female ovulates, there could be several donor sources available to unite with the female's eggs. For alot of owners, this is not a major concern, and then it becomes a guessing game as to which neighborhood dog is the "daddy". Not knowing who the sire is for a particular litter poses a problem for breeders. In that case, the puppies, mother, and suspected sires can be DNA tested. DNA test results are legally binding and can be used to determine lineage.