upon that individual with regard to genetic contribution in the absence of inbreeding . Pedigree information is useful for the selection of young animals prior to the development of their individual mature phenotypes . Pedigrees are also a means of identifying superior lines or families providing good records are maintained . A major disadvantage of pedigree selection is that pedigrees are often popular because of fad and not because of the phenotypic merit of the individuals it contains .
Considerable livestock selection is practiced on the basis of information or records of collateral relatives . Collateral relatives are related because of common ancestry rather than direct descent . The closer the relationship , the more valuable the information to selection procedure . Full sibs and half sibs are collaterally related and may provide much useful information for the selection of young animals . Selection by progeny tests means that we evaluate breeding value , primarily among sires , of individuals on the basis of the merit of the traits possessed by their offspring . It is very important that these comparisons be fair and accurate by using random samples of progeny . Selection of this nature is necessary for traits which are lowly heritable . Considerable selection of this kind has prevailed for many years in the breeding of Thoroughbred horses .
Most selection practiced in animal breeding is accomplished through sire selection . Horse breeders select for several traits simultaneously . This limits the amount of selection pressure which can be exercised on a single trait . We should place primary emphasis on those traits which are of greatest economic importance and have the highest heritabilities . Selection may be based on several criteria . Most young animals are selected initially on the basis of pedigree and records of close collateral relatives . Individuality becomes important at which time phenotype becomes mature or is actually established . Following reproduction , progeny information becomes available and selection continues on the basis of progeny . Good records on animals will materially aid all aspects of selection .
ANIMAL BREEDING PLANS
Animal breeding plans are systems which may be employed to facilitate greater control of heredity than if selection alone is used . These plans or systems are based on the degree of relationship existing between individuals which are mated for reproduction . Discussion of these animal breeding plans will be restricted to a within-breed basis .
The two main types of animal breeding plans employed by breeders are inbreeding and outbreeding . Linebreeding is mild inbreeding .
The mating of animals that are more closely related than the average of the breed is termed inbreeding . The parents of an inbred individual will ordinarily possess common ancestors as indicated on pedigrees extended to four generations . The mating of animals that are less closely related than the average of the breed is termed outbreeding . The parents of an outbred individual will not possess common ancestors as indicated on four generation pedigrees . If breeders plan matings with no regard to relationship , the plan ( or lack of plan ) comprises what is known as random mating . This method exists in nature but is seldom exercised by breeders because they nearly always give some attention to animal relationship in planning matings .
Inbreeding includes full sib matings and parentoffspring matings . Usually , no special effort is made to retain a high degree relationship between the inbred and one or more of the common ancestors of its parents . The plan may continue beyond a single generation .
The primary genetic effect of inbreeding is that of increasing the chance that an animal will inherit the same genes from both of its parents . This facilitates genetic homozygosity and prepotency . The trend toward genetic homozygosity facilitates the " fixing of traits ." The fixing of desirable or undesirable alleles is equal by chance under the influence of inbreeding . Selection can alter gene frequencies but inbreeding does not . Inbreeding accompanied by selection comprises one of the most powerful tools available to the animal breeder .
Rapid inbreeding over several generations is certain to cause some deterioration in size , vigor , and fertility . Many breeders avoid it , even to a slight degree . The success of an inbreeding program depends upon ( 1 ) the genetic merit of the foundation animals ; ( 2 ) rate or intensity of inbreeding ; and ( 3 ) possibilities and efficiency of selection . Inbreeding should usually be restricted to superior stock or herds . While it is practiced , the herd or line is closed to outside sources of genetic material which limits the germ plasm available for breeding improvement . While inbreeding presents some hazards , it is a highly effective animal breeding plan and has been used by outstanding livestock breeders who have achieved true breeding improvement . This is true in horses as well as other classes of farm animals .
There are no precise criteria for evaluating the advisability and extent of inbreeding prior to its adoption . A breeder must make these decisions as a result of trial .
Linebreeding is inbreeding , usually to a mild degree , practiced with the intent of maintaining a high degree of relationship of descendants to one animal of merit . The animal of merit will appear as the common ancestor of the linebred ' s parents . The plan is accomplished by maintaining a high degree of relationship between each parent and the common ancestor but otherwise not related to each other .
Outbreeding is the most prevalent animal breeding plan for livestock in general . Breeders employ it with the intent of introducing superior germ plasm into their herd from unrelated outside sources . Outbreeding increases genetic heterozygosity in a herd
and prevents the formation of distinct lines within breeds . Breeders who outbreed tend to follow the " random drift " of the breed with reference to the genetic merit of their herd . Because of its genetic effects , outbreeding facilitates reproductive efficiency , increased growth rate , and increased vigor .
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VOICE of The Tennessee Walking Horse