The importance and development of the Prepotent Sire

By | December 12, 2009

Every animal breeder knows the importance to breed improvement of a good sire who “throws himself” to a great extent. While a dam having this quality is valuable also, such a sire may have a much more far-reaching effect upon the strain and the breed as a whole, since he can produce a considerably larger number of offspring. When a stud dog stamps his likeness upon his offspring to a marked degree and passes on certain of his traits to an especially large number of his get or produces puppies that resemble each other more closely than usual, he is said to be prepotent. It must be remembered, however, that while an animal may be prepotent for one or more characteristics, this does not mean that he will also be prepotent for most or all other characteristics as well, Actually, propotency is not a property of an individual animal as a whole, but belongs to a few characteristics that are parent of the hereditary make up of.the individual. (Wentworth, “The Journal of Here-dity” 1915.) There are three major questions with regard to propotency that are of interest to the dog breeder. They are as follows:
1) What is the genetic basis of propotency?
2) How can prepotency be measured?
3) How can prepotent animals be produced by the breeder?

Genetic Basis of Prepotency: In his book, “Animal Breeding Plans,” Lush presents the answers to the foregoing questions. Much of the following material was taken from his book, He states that the most important genetic basis of a high degree. of propotency depends first upon homozygosity and second upon dominance. linkage and epistasis may also be involved in some in some instances. Homozygosity is the most important factor in propotency. A homozygous animal is one which possesses the gene for a given trait for both sire and dam and is said to be genetically pure for that characteristic. (Burns, “The Genetics of the Dog”) A completely homozygous animal would possess only one kind of gerrn cell and therefore he would pass on exactly the same genes to all his offspring. Any variation in genetic makeup would have to come entirely from the other parent. Dominance is also of major consideration in the explanation of prepotency. A dominant gene may be described as one of a pair of alleles (alternative forms of the same gene influencing the same trait but in different ways) whose effacts are expressed to the exculusion of the effects of the other alleles. (Burns) Every puppy receiving a dominant gene will show the effect of the gene, “if the gene is completely dominant and the parent is homozygous for it, then all of the off spring will appear exactly alike for the effect of the gene, regardless of the inheritance they received from the other parent. When a parent having many dominant genes is also highly homozygous, its prepotency is maximum.” (Lush). The effect of linkage is that of causing the offspring of an animal to fall in to fewer “type” groups than if linkage did not exist: The reason for this is that, in linkage, the genes concerned are located on the same chromosome (microscopic rod-like bodies which contain the genes). Two or more characteristics are transmitted togather nstead of assorting at random in the usual manner. Epistasis is believed to contribute to apparent prepotency in some cases. “When a gene of one pair of allelic genes masks the expression of the genes of another pair of allelic genes’ it is said to be epistatic to the other pair.” (Snyder, “The Princple of Heredity”).

Epistasis is similar to dominance except that (unlike dominance) the relationship is between non-allelic genes” (Burns) So a sire homozygous for one or more epistatic genes will produce a certain given trait or traits markedly in his offspring When mated to many bitches carrying a particular gene or genes. However, these genes may have little visible effect when the full combination is not brought together by the sire and dam, as when the bitch does not carry the complement that will complete this combination. The result of epistasis can often be that the offspring will look unusually alike but at the same time will be unlike either parent. The general effect of epistasis is to lower prepotency but in exceptional cases it may increase it. The Measurement of Prepotency The mos common method which may be used to measure prepotency is to note the resemblance of sire and puppies as compared with the resemblance ordinarily found between parents and offspring. When a prepotent sire is used, the resemblance will naturally be greater no matter what dam is used. The second method involves testing by comparison for a higher degree of similarity among the sire’s offspring, regardless of the dam, as against that commonly found between half brothers and sisters. However, there are those who would not call such a sire prepotent in cases in which the offspring, though very uniform, do not resemble him. Production of Prepotent animals: Although prepotent sires do occassionally appear in lines that are not inbred, the breeder’s chief aid in producing a prepotent animal is through the use of inbreeding. The reason for this is that while dominance, epistasis and linkage of genes are not within the power of the breeder to change or control, he can control the homozygosity of his stock. The more closely the mated animals are related, the more rapid will be the increase in homozygosity. Mating like to like without inbreeding does little to increase prepotency. Such sires show only slight prepotency when bred to unrelated or random bitches. In order to employ the valuable tool of inbreeding, the breeder must understand its meaning and function. “The broad scientific definition is that inbreeding is the mating of animals more closely related to each other than the average relationship with the population concerned.” (Lush) When practiced properly and wisely it is of great assistance to the breeder and through it vigorous prepotent animals of correct type are more rapidly produced. It is unfortunate that there is much lack of understanding among dog. A breaders as to what inbreeding does. Inbreeding does not create any weaknesses, defects or any of the physical or mental traits which will disqualify a dog of a given breed in the show ring. It merely exposes hidden recessives that have been present in the stock all along, under the shield of their dominant alleles, It does this by presenting them in a homazygous condition so that they are expressed and made visible to the observer. By it use the breeder is given an opportunity to discover exactly what heredity traits are carried by his animals. Also, in bringing these hidden recessives to light, he has a chance to reduce and, in some cases, completely remove from his strain those that prove to be undesirable. Genetically purer lines are thus formed from which the desired dog may be selected, producing a truer breeding strain. The case of the condition referred to by geneticists as “inbreeding degeneration” (early deat of young, lack of vigor and fertility) also demands clarification, for it need not be the result of extensive inbreeding. In an experiment by King involving more than 25 generations of rats at the Wistar Institute and in an experiment by Wright using more than 20 generationsof guinea pigs at the U.S. Department of Agricultre. “Inbreeding with careful selection has resulted in living, vigorous, fertile races even after many generations of brother sister matings.” (Snyder) Brother and sister mating is the closest possible form of inbreeding in animals.

Another aspect of the value of inbreeding is that some of the desired traits in a breed may be due to recessive genes rather than dominant genes. So by increasing the homozygosity of these wanted recessives as well as the wanted dominants, animals within the strain can be made to breed true for these characteristics also. Since some desirable traits are expressed by genes that are in the heterozygous (“Containing two different alleles of the same gene. A heterozygote produces two kinds of germ cells with respect to the gene in question” Burns) rather than the homozygous condition, intense inbreeding does have a drawback. When inbreeding is mild and selection intense. the heterozygotes can be readily maintained, When inbreeding is intense rather than mild, these heterozygotes can be reduced or lost from the strain. Mistakes that may be made by the breeder can be rectified, however. If the increase in homozygosity progresses too rapidly and a characteristic that is unwanted becomes fixed in the stock of the homozygous, condition, the error can be corrected by a mild outeross to a somewhat related animal that is carefully chosen. This can also be used to bring back into the heterozygous state genes which are desired to be in this condition. The success of a program of inbreeding depends upon the skill of the breeder and the careful selection and rigid culling of his breeding animals, based upon what they are able to produce (genetic makeup,) rathar than upon apparance alone, The ease with which this program may be employed and how rigid the culling must be depends upon the hereditary nature of the animals mated. Those possessing fewer genes for undesired traits (both hidden and apparent) need less severe culling and can be more closely inbred without the appearance of defects than those individuals or strains with many and varied “skeletons in the closet.” it is hoped that dog ‘breeders previously opposed to inbreeding of all forms will examine again its important function and endeavour to produce more prepotent strains. This system of mating has been used by livestock breeders with excellent success for the past 50 years. It is responsible for the development of a number of new and improved breeds of live stock and has significantly raised the quality of the old established breeds. As early as l9l5 the value of its ue was stressed in the journal of Heredity. Today the action and results of inbreeding are better understood that ever before. This has resulted in a more sophisticated approach to a breeder plan which has continued to be important over the years.

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