I totally agree with you, YR.

mfh27, why are you directly challenging Jenni? Why would her confidence about this topic not be warranted? She was agreeing with my statements, so why not challenge my confidence also?

We are talking about epigenetics here, the direct effect of environment on the DNA and RNA that causes changes in one generation from the next and can affect multiple generations. Genetics go way, way beyond dominance/recessive, heterozygous/homozygous and polygenes. Part of what epigenetics and RNA does is switches genes on/off and controls how extensively a gene may be expressed.
Take the longhair dog for an easy example, we've all seen longhairs that looked practically shorthaired, then there are moderately longhair and extremely longhaired dogs...they all are homozygous recessive longhair, but the RNA controls just how much that gene is expressed.
RNA is much more susceptible to changes by environment affects than DNA (though DNA is still certainly susceptible.)

Professor Randy Jirtle, well published in scholarly scientific journals, especially in epigenetics. One of his articles was on a study done with mice which showed that they could improve genetic health of the offspring by feeding the female mice a better diet. They paired up mice with the same genetic disease (in this case, mice that were genetically engineered to be morbidly obese and prone to diabetes and cancer and were short-lived) and just before breeding they improved the diet of the female mice by enriching it in methyl donors (for those of you who don't know, it's something commonly found fruits and veggies.) The diseased mice produced offspring that were exceptionally healthier; lean-bodied, not prone to cancer or diabetes and had longevity. The healthy offspring mice still carried the gene for obesity, but the gene was turned off.

An article published in the PNAS journel on a study led Dr. David Crews showed that when rats were exposed to a certain fungicide it caused changes in the expressions of certain DNA that affected neurological and behavioral changes for 3 generations removed from the original exposure. (that is, only the first generation was exposed to the fungicide and yet the deleterious affects were still being seen down through 3 generations of progeny.) The changes were not coded into the rat's DNA, but instead the RNA changed the expression of the DNA.

You can look up about the 2007 AICR conference and the various doctors and scientists who made separate presentations all showing research on how diet had direct epigenomical affect on the health of generations.

There are thousands more articles on research that proves environment has direct influence on genetics.

Here is a very simplistic explanation below; a very shortened version of a scientific theory I read some time ago. The following is not an end-all, be-all to having good genetics, obviously there are plenty of other factors.
It's a form of natural selection all on it's own. When a body is exposed to damages and poor environment then it assumes that it's environment cannot adequately support it's kind. A bloodline can semi-self-destruct, each generation that is exposed to negative environmental factors gives the next generation health problems (via the RNA turning on "bad" genes) to shorten the life-span and reduce the population. When a generation is given a healthy lifestyle and environment, then the body assumes that environment is ripe for populating and the RNA starts turning off the bad genes and turning on the good ones so that the subsequent generation(s) can live longer, healthier lives (and potentially produce more offspring.)

For those of you who read this post all the way through, congrats, I hope your heads are spinning, LOL.

There is also a theory that annual vacs may be at fault for so many allergies in dogs. I worked as a tech in the mid 70's to the late 80's and we saw plenty of dogs with allergies, mostly toy breeds. They weren't as common in larger breeds at that time.