P Smith
Lynn Whinery is correct. What has been done to the parents, grand parents and great grand parents has effects on the genetics they pass on. That's what epigenetics are. There are tons of articles on grandparents lifestyles affecting the current generation's genes. And vaccines are known to cause genetic mutations.
For a time it was common for only purebreds to be fed the 'best' commercial dog foods and get all the 'best' medications including the latest vaccines because they were worth something, where the common mongrel in the average home was left to fend for himself. (immunity wise) It is my belief that is why purebreds get the bad rap, they had more generations of disease causing agents behind them. When in reality they were the first canaries in the mine. Now a days the mix breeds are just as cherished as companions and have 'caught up' in a sense with the poor diets and over-medicated disease causing agents and as a result have the same types and amount of instances of illness.