Breeding isn't a simple as just puting two animals together and leting them do their thing. Geneticaly speaking there are 3 different types of breeding.
Outbreeding/Outcrossing: this is the most common type of breeding that is done and is basicaly breeding two animals that have no relation to eachother.
Linebreeding: Most of the best dogs come out of this meathod IMO, but it does require a great deal of knowledge in genetics, animal breeding, and the lineage of the line(s) that your breeding. This involves breeding dogs with similar lineage that aren't direct relatives, usualy cousins or something similar to that.
Inbreeding: Only the most experienced breeders should even consider this, if you don't know every single dog in the pedigree of the dogs to be bred, then don't even attempt to use this meathod. Inbreeding is breeding 2 direct realtive together, father-daugther, mother-son, brother-sister. Inbreeding is used to solidify a certain quality in a line. When it is done it's most comonly father-daughter, sometimes mother-son, and almost never brother-sister. Inbreeding can be very dangerous to the prospective pups and the female. Because of the genetic simialrities of the male and female, there is a high incidence of genetic mutaions in the pups, even resulting in things like the pups having 5 legs which will almost gaurentee that the female will have complications during the pregnancy and birthing.
If you don't want to breed your dogs, and you don't want your female to have saggy breast, then get your dogs fixed. There are many health benifits to having your dogs fixed and you'll have peace of mind knowing your dogs won't be running around trying to hump every dog they see
