Inbreeding, Linebreeding and Crossbreeding INBREEDING
's purpose is to fix certain traits or the influence of certain ancestors upon the progeny. This procedure varies in degree from intense closebreeding to mild linebreeding. Although inbreeding can be detrimental to fertility, vigor, and athletic ability within the offspring
it can also result in true-breeding strains of horses (that consistently pass important traits to their offspring). Because most breeds were formed by a process of inbreeding, the breeding of purebred horses is, my definition, a form of inbreeding. Some breeds are more inbred than others. (Degree of inbreeding depends on the number of common ancestors, how far back in the pedigree they appear, and how often each common ancestor occurs.)
From a genetic viewpoint, inbreeding results in an increase of the number of homozygous gene pairs in the offspring. Homozygous refers to a condition where two paired chromosomes have the same allelle (gene type) at a corresponding point. Because two close relatives tend to have more of the same alleles (by virtue of inheritance) than two unrelated individuals, their mating provides a greater chance for identical alleles to be paired within their offspring. This increase in homozygosity is directly related to the appearance of both desirable and detrimental characteristics that were not necessarily apparent in the sire and dam
When horses are inbred haphazardly, without culling of inferior stock, many undesirable traits may become predominant in their offspring. For example, the inbred horse's ability to resist disease and his overall performance capacity are often depressed. The growth rate of the inbred foal, and the average mature size within the inbred herd, frequently decreases. Nonselective inbreeding is directly related to a depressed fertility rate, an increase in abortion and stillbirth. Some basic principles of genetics show why these traits are directly related to inbreeding.
When two unrelated horses are mated, the chances of unidentical alleles combining within the resulting embryo are high. On the other hand, mating close relatives increases the pairing of identical alleles (increases homozygosity). The effect of increased homozygosity is a decrease in the number of heterozygous gene pairs and, subsequently, a decline in heterosis (i.e., loss of vigor and fertility). Although the reason for this allelic interaction is not clear, geneticists believe that its presence contributes to the overall quality of an individual. Therefore, as homozygosity increases within the inbred herd, physical quality controlled by overdominant alleles declines.
Many undesirable genes affecting the horse's overall vigor and fertility are recessive. Fortunately, they have no influence in the heterozygous state, since the effect of the recessive allele is completely hidden by the effect of the corresponding dominant allele. Because of the overall effect of inbreeding is an increase in homozygosity, it increases the number of homozygous recessives. Hence, the effects of undesirable recessive genes begin to surface. Inbreeding does not create undesirable trait, it exposes recessive alleles for hidden weaknesses which are present within the sire and dam. Because successful inbreeding demands the culling of inferior breeding stock over many generations (to help eliminate some of the undesirable recessive genes from the herd), it may not be feasible for some breeders. Not only is the time factor impractical for most breeders, the intense culling often necessary may be an economic problem. Additionally, the traits which tend to surface within the inbred herd (such as depressed growth rate and decreased size) contrast sharply with what many breeders select for. Therefore, the breeder must be objective when the need to cull arises.
Perhaps the greatest advantage of inbreeding is that it increases the prepotency of individuals within a herd and consequently helps to create distinct true-breeding strains or families. This prepotency (the ability of a stallion or broodmare to stamp desirable characteristics upon their offspring with a high degree of predictability) is the result of the parent being homozygous for important desirable traits. 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. (This is advantageous only if the parents are homozygous for desirable traits.)
As mentioned previously, inbreeding exposes certain weaknesses within the inbred herd. Uncovering these undesirable traits can be an important tool for the overall improvement within a large breeding program. By setting certain selection guidelines, and by carefully eliminating inbred individuals which show inherit weaknesses, the breeder can slowly remove any undesirable recessive genes from their herd. They will find that vigor and fertility are actually improved when inbreeding is accompanied by careful selection.A successful inbreeding program requires good foundation stock and severe culling over many years.
For this reason, inbreeding is usually practiced 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.
From the same source you quoted Druydess