Introduction
Genetic diversity in any species is important for long-term survival, both for the individual and for the species, or in the case of pedigreed dogs, for the breed. To understand why genetic diversity is important and how you can select for it in your breeding programme, read “Genetic Diversity Reporting in Litter Predict." Every time a breeder systematically selects dogs to breed based on certain characteristics, or an event occurs that results in only certain dogs reproducing, that population becomes both filtered and reduced. Specific breeding pitfalls are responsible for much higher rates of accelerated inbreeding. Some of these causes are outside our control, as they occurred decades or centuries ago, but some are choices that lead to contemporary lost diversity.
Use of popular or champion sires
Every dog carries a collection of defective genes, termed genetic load. When a sire is used repeatedly, his genetic merits and faults are spread widely through the gene pool, and although he may "fall off" the three-generation pedigree, his DNA contribution does not.
In addition, only a certain number of litters are produced annually. Selection of certain dogs to become the next generation of breeding stock not only results in some dogs having a greater influence, but it simultaneously excludes other dogs not chosen from contributing to the gene pool. This has been aptly compared to a juggling act; a ball dropped is a breed line gone from the gene pool, and with it, the unique features they carried.
Unequal use of males and females
Similar to the use of popular sires, when dogs are used to produce more litters on average than bitches, the unique lineage of the females in pedigrees are proportionally less represented in the future generations of the breed, reducing the gene pool. Equal use of males and females slows loss of genetic diversity.
Inbreeding to 'set type’
When linebreeding or inbreeding, particular lines are "doubled up" to produce more uniformity in offspring. Similar to popular sires and overuse of of males, this excludes other dogs from contributing to the gene pool and produces both immediate individual and delayed breed loss of diversity. It's worth noting that there are many breeds that demonstrate it is not necessary to line breed to achieve correct breed type.
Founder effect
When a small group of dogs is used as foundation stock for a new breeding population, this is called founder effect. The most common time this occurs is when breeds that were developed in one region or country become popular and are imported to another country. Because importation is expensive and difficult, only a selection of dogs is imported, usually with minimal subsequent flow of genes from the source population. Although the census count of dogs of that breed may increase dramatically over time, the "depth" of their gene pool does not increase.
The other main cause of founder effect is when studbooks or "book of origins" close, which for much of the western world was not until the mid-twentieth century. Closed studbooks do not allow registration of offspring from unregistered parents. By nature, breeds with closed studbooks will lose on average 3% breed diversity each year in a best-case scenario. Until studbooks were closed, dogs with one or both parents previously unregistered with the studbook of the registry could be admitted to the breed and recognised as pedigreed, provided they were of breed type.
Due to their limited genetic diversity, these foundation dogs do not completely represent the full diversity of the original population, and with time, develop different gene variant frequencies, called genetic drift, which may bring with it different traits or disease risks. The other very influential instances in which founder effect has impacted many western breeds was World War I and II, which caused the dramatic loss of dogs and breeds. Many breeds were re-established from remnant populations of less than 20.
The good news
Lost genetic diversity is unfortunately like interest on a debt – it compounds over time. However, there are measures breeders can take to mitigate diversity loss, such as avoiding popular sires and their close relatives, avoiding overuse of males, and refraining from overt linebreeding. Breeding to dogs from different populations, such as show to working lines, or from other countries harbouring unique genetics, can help. Use of frozen semen can also bring back genetic diversity that’s been effectively lost, in effect "rewinding the clock" on the breed. Litter Predict helps breeders know if a candidate mate to your dog is truly going to bring unique genetics, and therefore diversity, to your breeding programme’s offspring, saving you time and money searching for dogs more likely to bring "something new" to your kennel.