INBREEDING AND DIVERSITY – PART 4

Continued from PART 3

Figure 6. Seven Generations From an Outcrossing Event

1. “Outcross” represents the individual(s) of another breed
2. “Pure” represents a purebred individual of the limited-numbers breed

   (Read the above chart as: “Outcross” bred to the first “Pure” gave us #1;
   #1 bred to another “Pure” gave us # 2, etc.)

We are going to use a table of eight generations (“Outcross” through 7) to show the breed composition of a mating between any two individuals an arbitrary number of generations from the outcross event. Using the rules for computing a table of relationships presented earlier, we end up with the following table:

The lower cells can be filled from the upper cells. The entries in this table are relationships between the individuals in the pedigree, as we have seen before.

We can construct from this table a second one that contains the breed composition, in terms of outcross percentage, between matings. In the following table, the entry 2-3, for example, is the percentage outcross in a mating between a second and a third generation animal. The value is computed as ½ (ROutcross-2 + ROutcross-3), where the RXY are taken from the above table.

The table entries are presented as decimals rather than fractions because it is easier to read off values that way. Our intuition tells us that as time passes, the influence of the outcross upon the breed will diminish, and this is confirmed by the table above. There is no column or row corresponding to the outcross because only one outcrossing event was permitted in the outlined breeding program. When reading this table it must be noted that these matings are based on the assumption that the outcross event occurs only once in each animal’s pedigree. If there has been inbreeding in the population, the correct table may be computed from the appropriate table of relationships.

It is up to the individual breed associations to establish standards for breed composition, but we can provide a tool for studying possible rules. The following table is an excerpt from the second table above. The shaded portion of the table represents matings that are not permitted by the proposed breed association rules presented earlier. We are interested in answering the question: “Are these rules based on sound scientific principles?”

Our answer is a firm “No.” We can assume that the reasoning behind the mating rules was to limit the influence of the outcross and preserve distinct breed characteristics. One way to do this is to limit the number of times the outcross may appear in an individual’s pedigree. The obvious problem here is that there are permitted matings with levels of outcross composition much higher than some of the forbidden matings. This is quite obvious when considering the 2-5, 2-6, and 2-7 combinations. A more reasonable approach would be to limit the percentage of outcross, and not issue papers if that limit was exceeded. Such a decision might have to wait until there are several matings of each combination to determine if there is a natural threshold between acceptable and unacceptable levels of composition. We would recommend the establishment of an arbitrary criterion at the beginning of an outcross program that would be reevaluated at set points in time. For example, it might be decided that the outcross percentage should initially be limited to 15% or less. After four generations including the outcross, that level might be changed to 10% or less based on observations of composite individuals.

Outcrossing can be used to great benefit when a population is having a hard time maintaining a healthy breeding population. There are procedures, outlined above, for determining the outcome of specific matings to outcrossed individuals that can be used to establish breed rules for purity. There are no hard and fast rules for what those breed regulations should be. What can be stated strongly is that breeders should not be afraid to use outcrossing as a management tool. A healthy, vigorous dog is much more desirable than a “pure” one.

Putting it all Together

We have discussed some important ideas and introduced some useful tools for the dog breeder in the preceding sections. We will now conclude with a discussion of how to design and manage an effective breeding program. It is not enough to simply mate good animals to one another and hope for the best, especially when such important traits as show-ring success are very strongly influenced by environmental factors.

Bourdon (1997) discusses what he called common sense animal breeding. He emphasizes traits common to effective and successful animal breeders that bear reiteration here. Qualities desirable in a breeder include knowledge of theory and technology, patience and deliberation. The most important tool used by the careful breeder is good information. Finally, a good breeding program is characterized by consistency and simplicity.

Technical knowledge is important for an animal breeder. To be most effective, he must understand not only the methods he uses, but also something of the theory behind them. One does not have to be a mathematician or statistician to effectively breed good dogs, but should have a command of certain fundamental knowledge. The breeder has to understand the idea of Mendelian inheritance, for example, before he can properly understand how methods of computing inbreeding and relationship work. Another important concept to master is that genetic inheritance is a random mechanism that presents both opportunities and limitations. When the knowledge is available, the breeder should be aware of positive and negative genetic correlations among traits under consideration. If height is negatively correlated with litter size, and you are interested in fecundity, you should choose the short dog. Genetic mechanisms will not change for our wishing them to. Familiarity with the current publications and interaction with other animal breeders is very helpful in acquiring and maintaining technical knowledge.

While patience appears second in the list, it is perhaps the most important characteristic of the successful breeder. Genetic change occurs more slowly than many of us might like, but the trade off is that genetic gains are cumulative. The random nature of gene segregation makes much of inheritance unpredictable, so the wise breeder will play the averages. Adherence to a well-defined breeding program will produce successive generations that are better than their predecessors. Most offspring produced are considered average, but occasionally a truly outstanding individual will be produced. That individual should be patiently sought after, recognized and exploited.

No breeding program can succeed if it does not have clearly considered and realistic goals. The deliberate animal breeder will spend time thinking about his goals, his definition of the ideal animal, and the best way he can obtain animals with desirable characteristics. The temptation to jump on the bandwagon and follow the lead of other breeders should be resisted. A thoughtful manager will always know more about what is best for his program than anyone else.

The importance of good information cannot be stressed enough. Data about his animals is the single most important commodity any animal breeder possesses. Dog breeders do not have access to the kinds of information many livestock breeders do, typically genetic evaluations based on vast amounts of data and complex statistical analyses. Look for the most meaningful available. If pedigrees are the basis for most of your decision-making, do not accept incomplete or suspect pedigrees, and deal with “seedstock” producers (breeders) known to you to be of superior integrity and knowledge.

The nature of the information collected is also of interest. If it is not economically unfeasible, things like litter size, birth and mature weights and body dimensions should be collected, recorded, and contributed to breed databases. It is no disadvantage to breeders to share their hard-won information with others. When good quality data is available to everyone, the whole breed benefits. An example of the importance of collecting and sharing information might involve the discovery that a given line was a carrier for a deleterious recessive. The recognition of the problem would lead to breeders making more informed decisions about matings, particularly matings involving relatives. While the goal of a breeding program is the production of animals as close to ideal as possible, it does not relieve breeders of the responsibility of considering animal welfare.

The quality of information collected is of seminal importance. It really does not matter if there are thousands of records in breed books of they are not accurate records. You cannot make rapid genetic progress without sound information on which to base your breeding decisions. If you are faced with a tradeoff between the quality and the quantity of records you can record, always choose high quality.

The breeding program itself should be simple and consistent. By simplicity we mean that goals are carefully thought out and are reasonable in light of the information available and the genetics of the traits under consideration. Consistency implies that once goals are formulated, breeding strategies that lead to those goals are followed rigorously. That is not to say that there is no room for change in a breeding program; there is simply no room for hasty or ill-considered change. Frequent changes in breeding goals often result in contradictory efforts that lead nowhere fast. Clearly the patient animal breeder is better able to stick to goals once they have been established.

It can be difficult to formulate simple goals. There is often a temptation to try and improve several characteristics at once. Sometimes there will be positive correlations between traits of interest, and progress can be made in more than one area at once. However, you cannot change everything at once. There is a classic rule in animal breeding that states if you select for n traits, progress will only be 1/Ön (one divided by the square root of n) as rapid as if you select for a single trait. If you select for two traits, you only make 71% as much progress as if you selected for a single trait, and 58% as much progress if you select for 3 traits. The old maxim of “keep it simple” should always be borne in mind.

While there exists no absolute set of rules to guarantee your breeding program is successful, there are general rules to play by. You should always thoroughly understand the goals you are trying to attain. You should understand as much as possible the genetic mechanisms underlying the traits you want to select for. And you should understand that there is, and there will always be, a lot of luck. You cannot change the rules, but you can stack the deck in your favor.

Continued in PART 5