- 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).
- 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 allele (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.
LINEBREEDING, the most conservative form of inbreeding, is usually associated with slower improvement and limited risk of producing undesirable individuals. It can involve matings between closely or distantly related horses, but it does not emphasize continuous sire-daughter, dam-son, or brother-sister matings. The main purpose of linebreeding is to transmit a large percentage of one outstanding ancestor's genes from generation to generation without causing an increase in the frequency of undesirable traits often associated with inbreeding.
Because linebreeding is not based strictly on mating closely related individuals (with very similar gene types), it does not necessarily cause a rapid increase in homozygous gene pairs. Consequently, it will not expose undesirable recessive genes as extensively as closebreeding. For this reason, linebreeding is generally a safer inbreeding program for most breeders.
- CROSSBREEDING is the mating of horses from different breeds. Crossbreeding may also be used to produce heterosis, the sudden increase in vigor and fertility caused by a sudden increase in heterozygosity. Because horses from separate breeds usually carry very different genotypes, crossbreeding causes a more extreme form of heterosis. The possibility of each parent contributing identical alleles to their offspring is remote. Heterosis from crossbreeding often appears as a sudden improvement in physical characteristics, such as size, endurance, disease resistance, etc. New breeds are sometimes established by crossing members of two or more breeds and carefully inbreeding the original crossbred offspring.
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