INBREEDING/OUTBREEDING | house-shaman

Translation: K. Mete Sözeri 2018

Inbreeding, Outcrossing, and Breed Evolution

Tufts' Canine and Feline Breeding and Genetics Conference, 2013
Jerold S. Bell, DVM
Department of Clinical Sciences, Tufts Cummings School of Veterinary Medicine, North Grafton, MA, USA

The standards for purebred dog and cat breeds have developed over time through selective breeding.
These standards may describe physical appearance, temperament, or working ability.

In the early days of a breed’s development, such standards were often unwritten and informal, passed down through experience and observation.

Later, as breeds became more established, formal breed clubs began documenting and publishing these standards.

Written standards are not static — they are periodically updated.
Sometimes these revisions are made to clarify descriptions of the breed, and at other times to adapt to changes within the population or society.

At the beginning of a breed’s development, the pedigree records may include individuals with unknown ancestry or dogs and cats selected solely for their compatibility or working ability.
These animals form the foundation stock of the breed.

Some breeds were developed through close linebreeding among a small group of individuals that displayed a particular phenotypic (behavioral or physical) trait.
Since the discovery of the original breed records, many of the early family lines used during breed formation have been discontinued due to the emergence of undesirable or harmful traits.
Only those lines that consistently produced the desired characteristics and descended from the breed’s primary foundation ancestors have continued to be bred over successive generations.

Some breeds, on the other hand, were created by crossing individuals from already established breeds.
These individuals were members of breeds that had already undergone the original selective breeding and refinement process.
Such newly formed breeds typically go through a period of expansion, during which their characteristics become more clearly defined and stabilized.

Pedigree records of breeds show that, following their initial formation, the breed typically undergoes a significant population expansion that is accompanied by an increase in the average coefficient of relatedness within the breed.
Examples of this pattern can be observed in the Birman cat and the Cavalier King Charles Spaniel breeds.

Coefficients of relationship indicate the genetic relatedness between an individual’s parents. The average coefficients of relationship within a breed population reveal trends in the breed’s evolutionary development.

These coefficients can be viewed in two ways:

The overall average coefficient of relationship across all generations, or
The average coefficient of relationship calculated over a specific number of generations.

Unless unrelated genetic material is introduced into the gene pool, the total average coefficient of relationship can only increase over time.

In an expanding population, where breeding pairs are less genetically similar than those of the previous generation, the average 10-generation coefficient of relationship may temporarily decrease. However, one of the main factors that drives the long-term increase in average 10-generation relationship coefficients is the Popular Sire Syndrome.

This occurs when a breed’s gene pool becomes concentrated around a few fashionable or frequently used sires, leading to the genetic erosion of other high-quality male lines and a loss of overall diversity within the breed.

Molecular genetic studies of canine chromosomal structure have revealed the presence of large haplotype blocks—identical chromosomal segments—that reflect the effects of inbreeding and genetic bottlenecks occurring during breed development. These studies also demonstrate patterns of linkage disequilibrium (LD) that are characteristic of each breed’s formation history (vonHoldt BM et al., Genome Res. 2011; 21:1294–1305).

Research on dog breeds has estimated that, on average, approximately 35% of genetic diversity has been lost through the process of selective breed formation (Gray MM et al., Genetics. 2009; 181:1493–1505).

Molecular genetic studies of wolf populations mirror the processes observed in domestic breed formation. Research on Finnish Gray Wolves has shown that early admixture with Russian wolf populations, likely due to historical migration, introduced significant genetic diversity. Subsequently, as mean relatedness coefficients increased, heterozygosity declined, and the number of family lineages expanded, the population underwent a marked demographic expansion consistent with an increase in effective breeding population size (Jansson E et al., Mol Ecol. 2012; 21:5178–5193).

The modern breeds of cats and dogs have undergone the genetic selection processes described above. They exist at various stages of expanding their breeding populations and gene pools. Some breeds may have small but effective population sizes and exhibit high levels of homozygosity. However, if their offspring remain generally healthy, these populations can grow and expand over time. Such breeds are considered to be in earlier developmental stages compared to those with larger and more established population bases.

Population growth is an important aspect of breed development and preservation. It allows for successive matings between individuals that are less related than those of previous generations, promoting the creation of new family lines and increasing genetic diversity. Conversely, population limitation is detrimental to breeding management, as it leads to the loss of genetic lines and overall diversity. Maintaining an adequate number of breeders and pairings is essential for ensuring vitality and long-term survival of the breed.

Irk yapılanmasının bir sonucu olarak, köpek ve kedi ırklarının yüksek homozigotluğu vardır. Bu ırk oluşumu doğasıdır. Homojigosite tek başına, yüksek genetik yükte zararlı resesif genler taşımadıkça ırklara zarar vermez. Bazı ırklarda azalmış yavru sayısı, artan neonatal mortalite veya akraba katsayılarında artış ile daha kısa ortalama yaşam süreleri ortaya çıkabilir. Bu "akraba depresyonu" etkileri, spesifik hastalığa neden olan spesifik zararlı genlerin homozigot ifadesine bağlıdır. Bu genlere ve fenotiplere karşı doğrudan seçim, ırk sağlığını iyileştirmek için gereklidir. Eğer ırk bireyleri daha genç ölüyorsa, bu bireylerde spesifik hastalıklar ortaya çıkıyorsa, ırk doğurganlık ile ilgili sorunlar gösteriyorsa, bu sorunlara yönelik radikal seçimler yapmak gerekir.

As a result of breed structuring, dog and cat breeds exhibit high levels of homozygosity. This is inherent to the process of breed formation. Homozygosity alone does not harm a breed unless it results in the expression of deleterious recessive genes with high genetic load. In some breeds, reduced litter sizes, increased neonatal mortality, or elevated inbreeding coefficients can lead to shorter average lifespans. These effects, often referred to as inbreeding depression, depend on the homozygous expression of specific deleterious genes associated with particular diseases.

Direct selection against these genes and associated phenotypes is essential to improve breed health. If individuals in a breed exhibit premature mortality, specific hereditary diseases, or reproductive issues, targeted and rigorous selection is necessary to mitigate these problems and preserve breed vitality.

Linebreeding and outbreeding are genetic tools that should be applied with specific objectives in mind. Breeders can employ different reproductive strategies for each mating, depending on the goals for the population. These strategies involve pairing individuals that are either closer than the population average (linebreeding) or more distantly related (outbreeding).

Linebreeding serves to concentrate the genes of specific ancestors, while outbreeding introduces new, unrelated genetic material. By strategically alternating between these approaches—sometimes linebreeding, sometimes outbreeding, or a combination in different lines—breeders can maintain the genetic health and diversity of the breed population.

The only way to reduce the frequency of deleterious genes in a population (while increasing the frequency of favorable genes) is through direct selection based on genetic testing and phenotypic evaluation. The rate and degree of genetic improvement achieved through selection is directly proportional to the amount of variation among individuals within the breed.

Relying continuously on a limited subset of the population reduces the ability to apply effective selective pressure for genetic improvement. Proper selective pressure requires genetically diverse individuals to ensure meaningful progress.

Some studies highlight the presence of homozygosity within breeds and call for selection to increase the frequency of rare alleles and haplotypes. Molecular genetic tools can identify these alleles, but in many cases, the phenotypic effects of increasing their frequency remain unknown.

Genetic selection aimed at improving quality and reducing undesirable traits can potentially decrease the prevalence of certain phenotypes. Selecting blindly without understanding the consequences can significantly reverse the progress of selection-based breed development.

Some breeds may experience significantly reduced fertility or possess a very high potential for genetic disease. In extreme cases, a “survival” SSP-type plan may be required, which could involve importing breeding stock or outcrossing with other related breeds.

However, most breeds do not face such extreme conditions. For them, careful selection aimed at improving the gene pool and maintaining genetic health is sufficient.

Key Takeaways on Breed Evolution and Health:

The effects of relatedness (homozygosity, large haplotype blocks, and increased linkage disequilibrium) are a natural consequence of breed formation.
Healthy breed gene pools require expanding or large, stable populations.
Breed health should be assessed based on regular health and reproductive research.
Genetic selection for breed characteristics should avoid disease-associated phenotypes.
Breeders should avoid overusing the same individuals, as this is the most significant factor limiting a breed’s genetic diversity.