INBREEDING AND DIVERSITY – PART 3

Continued from PART 2

FY = 0. This example emphasizes a point made earlier: inbreeding is dependent on relationship. In small breeds, it often happens that there are a few very influential individuals to whom most of the population is related. These elevated levels of relationship can make it difficult to plan matings free of inbreeding.

Let us say, for the sake of argument, that we are thinking about mating Emma to Y because we are using her in a linebreeding program. The goal of linebreeding, usually connoting a “milder” form of inbreeding, is to maintain a high degree of relationship and similarity to a desirable individual, and is usually carried out by mating that individual recurrently. The pedigree in Figure 5 outlines a possible linebreeding scheme based on the repeated use of Emma as a dam. The paternal side of the pedigree is the same as shown in Figure 4.

Figure 5. A Linebreeding Scheme based on Emma

The first thing we do is add a column and a row to the table that we will fill in with coefficients of relationship between Z, the offspring of Emma and Y, and the rest of the dogs in the pedigree. We can also fill in the Z-Z cell because we already know the relationship between Y and Emma is 3/8, giving Z a coefficient of inbreeding of 3/16 (18.75%). The normal procedure is then followed to complete the table, which is presented below.

The shaded row and column contain coefficients of relationship between Z and the other dogs in the pedigree. We can clearly see that relationships between these animals are rising quickly because of the ties back to Emma in three out of four generations. Many breed societies have rules that dictate how frequently the same animal may appear in a pedigree, perhaps four times in six generations, and those rules are based on this idea. However, you can now see that the influence of such an individual depends quite a lot on just where in the pedigree the repeat appearances are. A more sensible rule might be that animals with a coefficient of inbreeding beyond a certain threshold will not be issued papers. A second approach would be to restrict the average relationship to influential members of the breed. The American Jersey Cattle Club publishes what they call a coefficient of kinship (K) on their pedigrees. That number represents the average relationship between the pedigreed individual and a set of the most influential sires in the Jersey breed. A disadvantage of this approach, however, is that K cannot be computed using the tabular method or other simple technique, and can vary considerably depending on the definition of the “most influential” group.

Introducing Genetic Variation through Crossbreeding

Breeds small in numbers (such as the Shiloh, certainly the Chinook and Klee Kai) are sometimes faced with the need or temptation to introduce genetic variation from an outside source to keep their population viable. This is typically done using crosses between the breed’s base (small gene pool) and another breed deemed to be suitable. The decision of what breed to outcross with can be based on anatomical conformation, behavior, or some other characteristic important to the base breed. A concern is often to limit the influence of the new breed to maintain an acceptable level of “purity”, or breed composition. An approach to calculating the outcome of matings between base and outcross animals will be presented and discussed.

Outcrossing may be used to take advantage of a phenomenon erroneously known as “hybrid vigor”, more properly heterosis. The idea is that a cross between two populations that have each become relatively homozygous will produce offspring that are heterozygous at many loci. Research has shown that such crosses are often much heartier, healthier and productive than either of the parental lines. Heterosis is an effect dog breeders have known about and taken advantage of for many years.

[Note: technically, a hybrid is a cross between two species, such as horse X jackass, or bison X cattle; crossbred dogs or dog-wolf matings do not produce true hybrids. But we should recognize that the word is used, and consider the context, even though a wolf is just another breed of dog.]

A tabular approach can be used to determine levels of breed composition. By breed composition, we mean the percentage of base and outcross breeds in the improved population. For example, if you were to cross populations of German Shepherds and Labradors, the composition of the resulting breed would be 50% GSD and 50% Labrador. Rules for acceptable matings are often based on distance between the outcross and current generation of individuals, and the tabular method to be presented can be used to determine if breed association rules are based on sound genetic ideas or breeder preferences.

We are going to use as the basis for this discussion a question posed to the author regarding breed composition and association rules. The situation is as follows: because of concerns about small population size, a breed association wishes to outcross for a single generation to bring in some new genetic variation. The outcross individuals will be used only for a single generation, and matings between composite (mixed) individuals will only be allowed if they are a certain distance apart in generations from the outcross. The club rules as they currently stood stated that:

(A): 2, 3 and 4 may only mate with 5 and 6, and

(B): 5 and 6 may only mate with 2, 3, 4, 5, and 6.

Those numbers refer to distance, in generations, from the outcross event. A ‘1’ would be the offspring of the limited-numbers breed and the introduced “outcross” breed. We will use the pedigree in Figure 6 as the basis for our discussion.

Continued in PART 4