Anoelle, ALL the different litters of puppies born from the SAME PARENTS ,the sire and dam , are FULL SIBLINGS , just in the same way as all the individuals in each litter are FULL brothers and sisters too. So breeding one puppy from the first litter with a puppy from the second litter is exactly the same degree of incest as simply breeding two puppies from the same litter. Genetically there is no difference as they are ALL full sibs.. Incest is bad news generally because it brings together two copies of harmful recessive genes. But it is one way in which pedigree animals are founded I,m afraid. That is the reason why all pedigree dogs etc suffer from breed specific diseases, through interbreeding of close relatives and full siblings are very close relatives. So I would advise against mating a dog with either a same age full sib. Or an older or younger full sib as explained. Why not have both parents neutered and help the dog population explosion. Cheers.
