“The indirect effects of selective breeding for appearance include very significantly reduced genetic diversity unevenly spread across the genome, resulting in elevated prevalence of specific diseases within particular breeds. Coupled with ill-advised breeding practices and insufficient selection pressure on health and welfare, this has led to certain breeds becoming especially susceptible to a whole suite of disorders, many of which are acutely painful or chronically debilitating…Since many diseases are the consequence of homozygosity for recessive alleles, breeding of close relatives is accompanied by a corresponding increase in the occurrence of these disorders…because an animal must inherit one defective gene from each parent in order to develop the condition. When parents are closely related, the liklihood of them both carrying a copy of the same deleterious gene is significantly elevated.” (Rooney & Sargan 2009)

“Since their domestication, dogs have undergone continual artificial selection at varying levels of intensity, leading to the development of isolated populations or breeds. Many breeds were developed during Victorian times and have been in existence for only a few hundred years, a drop in the evolutionary
bucket. Most breeds are descended from small numbers of founders and feature so-called popular sires (dogs that have performed well at dog shows and therefore sire a large number of litters). Thus, the genetic character of such founders is overrepresented in the population. These facts, coupled with breeding programs that exert strong selection for particular physical traits, mean that recessive diseases are common in purebred dogs and many breeds are at increased risk for specific disorders.” (Ostrander 2012)