This procedure varies in degree from intense closebreeding to mild linebreeding. 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.
From a genetic viewpoint, inbreeding results in an increase of the number of homozygous gene pairs in the 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.