Roots To Branches
INBREEDING is used to fix traits of certain ancestors upon the resultant progeny, varing in degree from intense closebreeding to mild linebreeding. Although intense inbreeding can be detrimental to fertility, vigor, and athletic ability within the offspring, with careful use it can also result in true-breeding strains- that consistently pass important traits to their offspring.
Because most breeds were created by inbreeding, the breeding of purebred animals is in itself a form of inbreeding, however some breeds are more inbred than others. The amount of inbreeding depends on the number of common ancestors, how far back in the pedigree they appear, and how often each common ancestor occurs. For instance, the Morgan horse is founded upon the characteristics of one horse in particular. All the original founders of the breed trace to this _one_ horse. Females of all type were used to cross to his many sons, however only three main sons come down to us today. Due to the "male-centric" views of the breeders at the time, his daughters often had no name and were only known as "bay mare by Justin Morgan", further clouding the ability to determine the actual amount of inbreeding in a particular individual.
From a genetic viewpoint, inbreeding results in an increase of the number of identical gene pairs in the offspring. Because two close relatives tend to have more of the same gene pairs (by virtue of inheritance) than two unrelated individuals, their mating provides a greater chance for identical genes to be paired within their offspring. This increase in sameness leads to a decrease in diversity and the appearance of both desirable and detrimental characteristics not necessarily apparent in the ancestors.
When animals are inbred without regard for undesirable traits these may become predominant in their offspring. For example, the inbred animal's ability to resist disease may be depressed. The growth rate of the inbred offspring, and the average mature size within the inbred group, frequently decreases. Nonselective inbreeding is directly related to a depressed fertility rate, an increase in abortion and stillbirth.
Some basic principles of genetics can show why these traits are directly related to inbreeding. When two unrelated animals are mated, the chances of unidentical alleles combining within the resulting embryo are high maintaining diversity. On the other hand, mating close relatives increases the pairing of identical alleles - decreasing diversity. The effect is loss of vigor and fertility. I had a female malamute who traced in ten generations to the same two parents on every line with only two outcrosses at the very beginning. This was severe inbreeding with little careful culling and in a breed known to be 100#(+) this female weighed only 45#.
Fortunately many undesirable genes affecting an animal's overall vigor and fertility are recessive and they have no influence in the diverse state, since the effect of the recessive allele is completely hidden by the effect of the corresponding dominant.
Because of the overall effect of inbreeding is an decrease in diversity, it increases the number of recessives. Hence, the effects of undesirable recessive genes not previously in appearnce begin to surface. Inbreeding does not create these undesirable traits, it simply allows the opportunity for the combining of recessives of hidden weaknesses which are present within the ancestors. Alaskan malamutes are prone to a dwarfism disorder and must have a pedigree analysis to deterime the % of dwarfism genes present. An animal with a high % which is intended to breed anyway must have a testbreeding to a known carrier to see if offspring of the cross are born with the disorder.
Since a successful inbreeding program demands the culling of inferior breeding stock over many generations (to help remove some of the undesirable recessives from the group), it generally not be feasible for most breeders. Not only is the time factor impractical, the intense culling necessary is an economic problem as well. Additionally, the traits which tend to surface within the inbred group (depressed growth rate and decreased size) contrast sharply with what many breeders select for. The breeder also needs be truthful with themself and maintain objectivity when the need to cull arises regardless of the financial or the emotional costs- "...that baby cost a bundle in stud fees" or "...but I NEED a daughter from this cross".
Perhaps the greatest advantage of inbreeding is that it increases the predicatability of characteristics within a group and helps to create true-breeding strains or families. This "prepotency" is the result of the parent being homozygous for the chosen desirable trait. When such a parent carries two identical alleles on corresponding points of a chromosome pair, he transmits that allele to the same chromosome point within his offspring. If two such parents are mated, the offspring will always possess the same desirable trait. Therefore, as inbreeding increases homozygosity, it also enhances prepotency.
As mentioned previously, inbreeding exposes weaknesses within the inbred group. Uncovering these undesirable traits is an important tool for the overall improvement within a large breeding program. By setting strict selection guidelines, and by carefully eliminating individuals which show these inherited weaknesses, breeders can locate and slowly remove any undesirable recessive genes from their group. Vigor and fertility are actually improved when inbreeding is accompanied by careful strict selection.
A successful inbreeding program requires good foundation stock and severe culling over many generations. For this reason, inbreeding only practiced effectively by experienced breeders who operate large farms for the production of superior prepotent breeding stock. It can also be used to establish breeds, or true-breeding types, with respect to certain characteristics such as color or size.