Mode of inheritance
Even though it doesn't take a rocket scientist to breed quality dogs it does take a learning process and probably as many years of study... like a geneticist there is a need to know the dog's/bitch's background, genetic make up, and or family tree a good mentor comes to mind for the beginners ... through trial and error or scientific research its all heading for the same goal in mind... a healthier dog. Now with that little tid bit noted on to mode of inheritance or my opinion on how some things are inherited.
Say, I have a litter born with one heart related problem but the rest of the litter has no issues at all... there are 4 pups in this litter.... because there was only one pup inflicted I would almost think it was a fluke of nature.... being that 50% of the genes came from a healthy Sire and the other 50% coming from a healthy Dam. With this knowledge I would then break it down to 25% afflicted, 25% non carriers, 25% carries and the other 25% up for grabs. I would also keep close tabs on the other 3 non afflicted. Then remove the heart problem from the gene pool as a non breeder/pet which would not be sold... which now leaves me with 3 healthy pups. Then by culling again from show to pet placing the pet pups in a pet home with No breeding permitted. Breeding only my show stock, keeping close tabs on any and all health problems that might Adminse when these pups mature, my last stage of culling before breeding. After that keeping a close watch on any other litter out of the one or two healthy pups allowed to be bred. E's favorite saying... "Don't throw the Baby out with the bath water"... lol
Now, lets say again you have a litter of 6... 2 or 3 have heart problem and using what I said above would you think this litter gives a clearer picture on what's inherited? To me the answer would be yes. While one pup in a litter of 4 could be a fluke, environment, or from previous use of medications that interfered with the development of the fetus... having more pups in a litter with the same problems that would defiantly throw up all sorts of danger signs... No matter how it is inherited through the sire, dam or both ... this breeding should never be repeated. Also may take some research on your part. Your mentor should be of help here remembering the Dogs from the past as well as his own breeding program. I can't tell you how invaluable this information is.
Please put any health problem in place of the "heart issues" and it should work out the same.
People there is a lot more that goes along with this bit of information but it's a start...