Maintaining Genetic Diversity: Why it Matters

It may seem dramatic to claim that many dog and cat breeds, including some very popular ones, could be classified as endangered or critically endangered, but Casey Knox and Katie Lytle present a thoughtful discussion on genetic diversity – or the lack of it – in our pet population, and why it matters.

What is genetic diversity?

The astonishing variety seen within the estimated 400 domestic dog breeds worldwide is a product of, and testament to, their intimate relationship with human development over the last 14,000 years and the selective breeding that has taken place in that time. Consider that the 2 lb (0.9 kg) teacup Chihuahua and the 200 lb (91 kg) Great Dane exhibit a 100-fold size difference, the largest range observed within a mammalian species, and we can begin to understand the degree of impact that humans have had on the domestic dog. To date, 19 million unique genetic variations have been found across the canine genome (1). The pedigree domestic cat, on the other hand, shows less variability and a shorter history of intentional human breeding; there are approximately 70 recognized cat breeds, and the majority of these were only developed in the past 80 years. In both dogs and cats, however, relatively few gene variants, or alleles, are responsible for the variety of physical traits humans have carefully propagated through breeding, compared to the overall genetic diversity found within each species. 

We know that in many things “variety is the spice of life” and research has shown that genetic diversity within a species is no different. When we think about the genetic diversity of a population, we consider the variety of genes present within a population in its totality. This includes alleles influencing physical appearance as well as biologic processes (see Figure 1). In contrast, within an individual we describe genetic diversity as internal diversity, hybrid vigor, or heterosis. Diversity can have direct and profound impacts on population health and long-term survival. Zoo conservationists are well aware of these risks, and thus have developed advisory groups and species survival plans for many of the animals in their care, with the goal of cooperatively working together to maximize genetic diversity, appropriately managing the demographic distribution and long-term sustainability of a species or subspecies (2). When we consider our companion animals from this viewpoint, we come to realize that our dog and cat breeds can be thought of in much the same way, as they represent isolated populations with a limited number of individuals, reared and bred primarily in captivity.

Genetic diversity is the resource or “tool library” on which a population relies when faced with a new challenge, whether that is a maladaptive DNA mutation, exposure to a novel virus, or an environmental challenge. The most obvious benefit of a more diverse gene pool is a reduction in the likelihood of recessive, maladaptive mutations pairing up in each generation, manifesting as disease. We know from the 1,000 Genomes Project that maladaptive mutations are present in every human; this is termed “genetic load”. Researchers have found that the average human has 50-100 disease-causing mutations, as well as 250-300 loss-of-function mutations (3). It is reasonable to assume that dogs and cats would also, on average, carry maladaptive mutations, and recent research has supported this idea. In a study of nearly 7,000 purebred dogs, representing 230 breeds, each animal was tested for 93 risk- associated variants. The researchers found that 17.8% (N = 1,208) of the dogs carried at least one of the tested variants, while 2.5% (N = 170) were genetically affected for a tested disease (4), a result that challenges the idea that genetic disease- associated variants in our canine population are rare. The genetic load of disease-associated mutations is not limited to purebred dogs however. Following the evaluation of nearly 35,000 mixed breed dogs for 13 disease mutations, a separate study found that two of the mutations were detected at a high enough frequency that the “assumption that mixed-breed dogs do not suffer from single- gene genetic disorders is shown [...] to be false” (5). [...]