"Why not produce traits by direction instead of by chance"
The term pedigree is an old word which is derived from the French "pie de grue", meaning crane's foot. The drawn pedigree was first used in the breeding of cattle and other livestock. Now after more than six centuries, the tradition of using it as a primary breeding tool continues. Over these many years breeders have learned that there are many kinds of pedigrees and many ways to use them. The traditional pedigree is the most popular of all the pedigrees because it focuses only on the names and titles of each ancestor. Unfortunately, as a breeding tool it has many shortcomings. The most notable of which is the importance it places on knowing the names and titles of dogs, which are not heritable. Secondly, it requires breeders to recognize and associate names and titles with what can be remembered about each ancestor. Its third weakness is in its lack of reliability for capturing the information needed to plan breedings. There are other problems associated with the traditional pedigree. For example, when something worked, credit was given to the pedigree and the breeder. When it didn’t, there was no record or source of information that could be reviewed. Finally, it was ineffective because it relied primarily on the use of trial and error breeding methods. Perhaps its major criticism was that it did not lend itself to collecting the right kinds of information in sufficient detail to be useful to plan a breeding of the better dogs.
On of the most important uses of any pedigree is in its ability to identify the carriers along with the strengths and weaknesses of the ancestors. Thus, when the frequency of a trait or disease occurs with regularity among the ancestors it can be noticed as something that is heritable. A pedigree that allows breeders to search through the ancestors for problems, their qualities or the lack there of becomes a useful tool. Since there are many kinds of pedigrees, breeders need to use those that are best suited for each of their breeding needs or problems. More will be said about this in the discussion that follows.
Modes of Inheritance
The focus of this paper is a discussion about how the simple recessive mode of inheritance works and how breeders can identify carriers in their pedigrees. The simple recessive mode of inheritance means that both parents must each pass on to their offspring one recessive gene in order to produce a trait or disease. Another mode of inheritance is called the polygenetic (many genes) mode of inheritance. It means that either or both parents can be involved. Examples of polygenetic diseases are: hip dysplasia, diabetes, hypertension, autoimmune, kidney disease, aging, etc.
To illustrate how the simple recessive mode of inheritance works the Symbols Pedigree will be used. The Symbols Pedigree is unlike the traditional pedigree in many respects. It is the pedigree of choice used by researchers whose work is centered on identifying the carriers, affected and the normals. The Symbols Pedigree gets its name because symbols are used to code gender. A circle is used to designate the females and squares the males (see Figure 1). Each symbol is coded with a specific color to identify each trait and disease. When an individual is known to have a certain trait or disease, its symbol is shaded (coded) with a specific color to designate that trait or disease. Carriers of the same trait or disease are coded using a dot of the same color that was used to note the presence of the trait or disease. (Battaglia 1986). Most breeders will use several colors when coding the traits and diseases found in their pedigrees.
One way to learn more about each sire and dam is to extrapolate from the pup's they have already produced. For example, Figure 1, illustrates two different breedings. In each breeding there occurs one female pup ("g" and "i") with progressive retinal atrophy (PRA). This is an eye disease that leads to blindness and is common in many breeds. PRA occurs when two carriers each pass one recessive gene for PRA on to their offspring. In other words, both parents must be carriers of the gene in order to produce PRA.
In practice, most breeders do not know if their dogs are carriers for any disease. This causes them to breed using trail and error methods. But once a few pups have been produced the carrier status of the parents and their littermates can begin to be determined. For example, we know that the mode of inheritance for PRA involves simple recessives. This means that when just one pup occurs with PRA both parents are known to be carriers of PRA. In each of the breedings shown in Figure 1, (a X b and c X d) there is one offspring produced with PRA (g and i). These affected offspring confirm that their parents were carriers of PRA, which explains why the symbols for both sets of parents have been coded with a red dot. Analysis of these pedigrees should not stop here. One should also question the carrier status of the remaining offspring in the second generation (e, f, h, and j) and all of the pups (k through p) in the third generation. The rationale for this concern can be found by using the Punnett Square. The rule for using the Punnett Square is straightforward. Upper and lower case letters are used to code all of the possible alleles carried by the male (sperm) which are arranged along the top of the square. All of the possible alleles carried by the female are listed on the side of the square. By combining the letters for the alleles carried by the male with those carried by the female, all possible combinations can be predicted (Battaglia 2003). For the sake of convenience, the recessive alleles are represented by lower case letters (b, w, etc); the dominant alleles are represented by upper case letters (B, W, etc). The careful selection of sires and dams can also influence the possibility of producing more carriers. Notice in Figure 2 that when carriers are bred to each other the frequency of carriers increase to 75%.
Understanding how this mode of inheritance works is important because it can be applied to other traits such as coat color and length of coat. For example, imagine that two black dogs are bred. One is black because it carries two dominant genes for black (BB); the other is black because it carries one dominant gene for black and one recessive gene for the recessive color liver (Bb). Notice that in Figure 3, when a carrier is bred to non- carrier, 50% of their offspring are expected to be carriers even though none of the pups will exhibit the recessive trait. The recessive trait will stay hidden and only the dominant trait will be exhibited. This means that all of the pups will be colored like their parents. The problem for the breeder is learning how to identify the carriers. This can be done by following the offspring as they mature and are themselves bred.
By keeping records on a Symbol Pedigree the carrier status of each pup and each ancestor can be estimated or determined. One advantage gained from using the Symbols Pedigree is that it produces a visual picture of
the information collected.
Backing into a pedigree by extrapolating from what is observed in the pups is one of the best ways to understand the carrier status of the ancestors. Because the recessive genes can be present but not expressed requires that breeders discipline themselves to follow their litters as they mature and are themselves bred. There are four important principles to be remembered. First, recessive genes can skip one or more generations before they appear. Second, whenever individuals on both sides of a pedigree are known to carry or display the same trait, the chances of it appearing in their pups increase. (See Figure 2). A common mistake is to not select against a carrier because it has not yet produced an affected pup. Third, by not selecting against the carriers a breeder will cause the frequency of carriers to increase to 50%, which is higher than the average for most breeds. (See Figure 2). On the other hand, eliminating all of the carriers from a breeding program quickly affects the diversity of the breeds gene pool and many of the better dogs will be lost. A better approach would be to use dogs that are likely to decrease the frequency of carriers. This is called managing the carriers and involves the careful elimination of carriers in successive breedings. (Bell 2000). Fourth, recessive genes are not all bad. In many breeds the recessive genes produce many of the desirable traits of conformation. Table 1 shows a few of the conformation traits produced by simple recessives. While this list could obviously be made longer, it serves to illustrate what breeders can easily control by direction.
|German Shepherd Dog||White, blue, liver, black||Long|
|Chinese Crested||Power Puff|
Each of the traits listed in Table 1 are controlled by the simple recessive mode of inheritance. Breeders who use the Symbols Pedigree as their record system coupled with the principles listed above and illustrated in Figures 2 and 3 can more quickly produce traits by direction rather then by chance.
In the final analysis, how much of a difference a breeder will make in his or her breeding program will be directly related to how well these principles are understood. For a more in depth discussion on how recessive genes can be passed from parent to offspring, refer to the article entitled "Punnett Squares" which is listed in the references.
Battaglia, C. L. - "Punnett Squares", Canine Chronicle, Vol. 27, No 8. Pg. 188-120, 2003
Battaglia, C. L. - "Breeding Better Dogs", BEI Publications, Fifth Edition, Atlanta, GA 1986
Bell, Jerold S. "Choosing Wisely", AKC Gazette, August 2000, Vol. 117, Number 8, p-51.
Bell, Jerold, S. "Developing Healthy Breeding Programs", Canine Health Conformance, AKC Canine Health Foundation, Oct. 15-17,1999. St. Louis MO.
Foley, C.W; Lasley, J.F. and Osweiler, G.D., "Abnormalities of Companion Animals: Analysis of Heritabliliy", Iowa University Press, Ames, Iowa, 1979
Hutt, Fred, "Genetics for Dog Breeders", WH. Freeman Co., San Francisco, CA, 1979
Willis, Malcolm, "Genetics of the Dog", Howell Book House, New York, New York, 1989
Willis, Malcomb, "Breeding Dogs", Canine Health Conference, AKC Canine Health Conference, Oct. 15-17, 1999. St. Louis, MO.
Willis, Malcomb, "The Road Ahead", AKC Gazette, August 2000, Vol. 117, Number 8, p-47.