Introduction

In the "ideal" cases dealt with previously, once a population of interbreeding animals is assembled, Hardy-Weinberg equilibrium is established after one generation of random mating. Thereafter, the proportions of animals with the different possible genotypes at the locus in question remain the same, generation after generation. It is important to recognize that this consistency depends upon unchanging allele frequencies. Clearly, allele frequencies can change over time within a single population, and frequently differ between populations. The following discussion deals with the most important factors affecting allele frequencies: Genetic Isolation, Migration (gene flow), Mutation, Natural Selection, Artificial Selection, and Chance.

 

Genetic Isolation

Genetic isolation refers to the separation of a potentially interbreeding population of animals into two or more groups that do not exchange genes. While reproductive isolation does not in itself change gene frequencies, it is often a precondition to such changes. Genetic isolation can occur as a result of geographic barriers, as was the case in the early development of the Channel Island breeds of cattle, the Jersey and Guernsey . In more recent times, isolation occurs because of the pedigree barrier. In the early history of most breeds of domestic animals, interbreeding between groups was not rigorously controlled, but once the herdbook or registry was closed, any animal registered as a member of a "pure" breed was required to have parents that were both registered members of the same breed.

 

Migration, or Gene Flow

Alleles can flow from one population to another when animals migrate and begin to interbreed in new localities, or when there is deliberate crossing of breeds or subpopulations within breeds.

 

    Cross-Breeding

When new stock is introduced into a breeding population, there may be a change in gene frequencies, particularly if the animals introduced have markedly different allele frequencies. In livestock, the usual situation is the deliberate introduction of animals with differing genetic constitutions for the purpose of introducing particular genetic traits (e.g., the polled condition in cattle), or for "grading up". In a grading up operation, in which there is the importation of pedigreed animals or the use of imported semen, the objective may be to replace a part of the gene pool with alleles characteristic of a superior population. This is usually accomplished by an initial cross and a series of backcrosses.

Under the rules of registration for most purebred animals, the only offspring that can be considered bona fide members of a breed are those whose parents are already registered members. The gene pool is thus essentially closed and the breeder must work with the genetic variation available within breed at the time of closure, plus any additional variation that may arise by mutation. The amount of variation will vary with the size of the population and the breeding practices.

There are a number of ways in which genes can flow into the gene pool of a breed from other sources. Undetected accidental crosses between breeds have undoubtedly caused some changes in the allele frequencies of many breeds. Deliberate crossing of two breeds with subsequent backcrossing to one has been practiced for some time in Great Britain , where the British Kennel Club allows registration of offspring of the 4th backcross. Such dogs have, on the average, received 31/32 of their genes from the breed under which they are registered. The remaining 1/32 can be considered as different alleles from an outside source, but since all breeds share a proportion of their alleles, the actual genetic differences introduced by such crosses are probably considerably less. The major purpose of "crossing-in" genes from other breeds has usually been to increase the available genetic variation within a breed whose vigor and reproductive performance have declined because of inbreeding.

In the illustration below, Breed A and Breed B are crossed. The hybrid receives 1/2 of its genes from A and 1/2 from B. The hybrids are then backcrossed to breed B and the resulting offspring (first backcross) receive 3/4 of their genes from breed B. By the 4th backcross to B, 31/32 of the offspring's genes are derived from breed B. Note that if the diagram is viewed on its side with breed B at the top, the points representing various crosses form a curve of increasing contribution of breed B. The curve rises sharply at first, then more slowly as the proportion of genes derived from breed B approaches but never reaches 100%.

Calculation of Proportion of Genome from Breed B
AB Hybrid = 1/2
BC 1: (1/2)(1/2)+(1/2) = 3/4
BC 2: (3/4)(1/2)+(1/2) = 7/8
BC 3: (7/8)(1/2)+(1/2) = 15/16
BC 4: (15/16)(1/2)+(1/2) = 31/32

In a recent case, the first of its kind, Schaible attempted to develop Dalmatian stock lacking the autosomal recessive gene for high uric acid excretion (a cause of kidney stones) by crossing Dalmatians with English Pointers and then backcrossing repeatedly to Dalmatians. Selection occurred at each generation for low uric acid excretion. Although 2 low uric acid excreting dogs from the 4th backcross were registered as Dalmatians by the American Kennel Club, there was such an outcry from some ill-informed Dalmatian breeders about "contamination" of the breed with Pointer genes that the AKC suspended further registration.

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