Upgrading Considerations

The various philosophies for breeding and sheep management are as many and varied as are the number of flocks and shepherds today. The information contained here is intended to provide a discussion of the challenges and considerations inherent in developing animals through the use of importation and upgrading processes for Teeswater Sheep. There are specific nuances in every genetic population and situation, and the discussions here should be considered in that context. These statements are not intended to be absolute, and the concerns raised are not an indictment on the quality of any other program. It should be noted the competing demands of breed selection will shift over time as populations develop and one of the primary aspects of a breeding program should be recognizing the variability of selection  characteristics while still maintaining genetic stability in breeding populations.

By its very nature, development of purebred populations is genetically entropic. The selection of specific sets of characteristics and the ability to reliably repeat those characteristics naturally leads to the loss of other traits that are not associated with the ones favored by selection pressure. Further, the artificial selection pressure on traits not directly related to survival poses an inherent threat to long term population survivability. This is not to say that purebred development is intrinsically negative, only that careful consideration must be given to the long-term implications of various decisions on the permanent viability of the population being worked with.  

When one considers the various types of selection pressure, the single most significant pressure that exists in domesticated flocks is the artificial selection pressure created by humans in selecting various traits. Natural selection indiscriminately favors those traits or adaptations that makes an animal reproductively successful within an environment. These traits include things like disease and parasite resistance, prolificacy, feed efficiency, maternal ability and other considerations that allow animals to successfully survive and reproduce. Certain Regions of the world favor specific adaptations, as is evidenced by the various breeds worldwide. Some of these adaptations can be highly specialized and very elaborate or relatively simple. Artificial selection is human induced selection pressure that favors various traits with some perceived value, including things like fleece quality, carcass quality, exaggerated reproductive characteristics, appearance that may or may not have any sort of reproductive advantage.

Often this artificial selection pressure artificially exaggerates certain traits and weakens others, sometimes at significant detriment to the natural viability of a population. Carried to an extreme this can mean that certain populations are no longer viable without extensive intervention from shepherds in order to be sustained – not a particularly good long term survival strategy.

Purebred associations with small flock populations can be particularly susceptible to genetic drift, or skewed selection pressure because the small populations that exist are being subjected to similar and consistent selection pressure for specific characteristics. This can be exacerbated if a large number of those association members show competitively in the absence of well-considered and implemented breed standards.  Combined with the very limited genetic pool originally available, the high coefficients of inbreeding, and the use of few and arbitrary selection criteria, (ie the use of theoretical blood percentage cutoffs for registration purposes, which may lead to culling of phenotypically superior or genetically valuable animals) the Teeswater, especially as it exists in the US is one such vulnerable population.

Conventional association wisdom holds that multiple generations of linebreeding with imported or at least “higher percentage” genetic material on a small population of phenotypically similar animals will yield an effectively “purebred” US animal by the time a mathematical blood percentage of 96% has been achieved. On its surface it seems reasonable to copy the genetic information of an individual that has successful traits, however the copying process is imperfect, and the loss of vital genetic information along the way can remove diversity critically important to the breeds future adaptation and survivability in altered conditions than those at the time of the original copying. Additionally, animals below specific mathematical percentages are to be purged from the breeding population, irrespective of type, and registration restrictions are included for specific characteristics including Codon 171 and color. This approach relies heavily on several basic assumptions:

• The inherited characteristics “genes” from the purebred animal occurs at a rate of at least 50% for each breeding

• The genetic heritability of the traits of the imported breed are dominant or prepotent, meaning that it is expected that the genetic contribution of the dam line is considered subordinate or corollary to the animals genetic profile and is basically to be “overwritten” by successive generations of breeding up

• The effect of the underlying dam line is not considered in recombination in terms of the ability to eventually express the phenotype of the desired line, therefore the contribution of the underlying breed is not considered as being an intrinsic part of the “purebred” form.

• The genetic information of both the foundation stock and the imported breed are free of underlying or recessive defects.

Unfortunately, genetics are not nearly so straightforward. Heritability is highly complex, interrelated and dependent on several considerations, the animals that have been used in upgrading have foundational genetic similarities that skew mating results in unexpected and un-anticipatable ways, some good, some bad; and unfortunately no genome is free from defects. Fortunately, extensive study and research has been dedicated to the heritability of various traits and information exists to guide and assist breeders in developing individual strategies to manage all the competing interrelated considerations.

While US upgrading programs have been granted what is in effect a short reprieve from the devastating effects of permanently and irrevocably narrowing genetic pools to unsustainable levels through the contribution of foundation ewes, only a few generations of line breeding or inbreeding can result in losses of genetic diversity over 90%, this can be particularly problematic when bloodlines are not well studied and careful attention is not paid to tracking and selecting against defects, which can find their origin in both the imported and  foundation stock. Such is the legacy of the modern upgrading program. These issues are exponentially exacerbated in designer breeds in the race to purebred status, a stigma that the US Teeswater population suffers from, as mathematical blood percentage can and regrettably have trumped almost all other breeding considerations in many cases.

The TSS represents an alternate philosophical approach to the tools currently used by other Teeswater breed associations in evaluating the genetic value of upgraded animals, both in the context of phenotype, and in the genetic diversity they represent for the breeding population. This is not to say that line breeding, or in-breeding are uniformly detrimental or not justified and necessary, it is only to say that they have the effect of removing genetic information, and it is necessary to both understand and provide adequate data to breeders to understand both the existing genetic pool and the effect of breeding trends over time. It is also important to understand that even in its native form, the purebred Teeswater relies on the consistent and careful selection of its breeders to continue to maintain viability and quality. It is unreasonable to expect that indiscriminate reproduction of any population will not lead to animals that do not express ideal type and should not be perpetuated in the breeding flock.   It is important that as many tools and evaluation techniques as possible are available to breeders to fit their specific situation and issues and assist them in this ongoing analysis. This picture changes over time both within flocks and within breeds and the rules of a Society should not be at odds with breeders having the ability to make the best decisions for their flock.

The TSS  seeks to provide a way to protect genetic diversity, including natural viability, while ensuring consistency and adherence to the Teeswater standard of type. Additionally, the TSS seeks to provide breeders with information and tools that combine techniques explored by various breeders to provide tools that best fit the individual genetic and environmental challenges of a flock while still protecting the genetic population. In application, genetic diversity and developing a purebred population to a standard of type are diametrically opposed goals. It is perhaps more accurate to say the goal is to maintain the greatest degree of genetic diversity possible within a population that consistently exhibits the desired phenotype and avoid fixing undesirable traits that cannot be corrected. This definition seeks to balance the implied need to maintain the natural viability of a breed with the exaggerated selection criteria imposed by man.