Interbreeding refers to reproduction between animals that share close genetic ancestry — either within a family line (inbreeding) or between two distinct breeds or strains (crossbreeding, depending on context). In the horse industry the term most often describes selective breeding practices used to fix desirable heritable characteristics such as conformation, gait, or temperament within a closed stud-book constraints on genetic diversity line.
Controlled line-breeding — mating animals that share a common ancestor two or three generations back — increases genetic prepotency, meaning offspring more reliably inherit the target traits. However, as the coefficient of inbreeding rises, so does the probability of pairing recessive alleles that cause hereditary conditions; examples in horses include Hyperkalemic Periodic Paralysis (HYPP) in American Quarter Horse descendants of the a carrier stallion as the source of a heritable disorder Impressive, and Malignant Hyperthermia in some breeds. The American Saddlebred and other closed-stud-book breeds have historically used interbreeding to maintain type. Modern breed registries limit acceptable inbreeding coefficients to balance trait consolidation against genetic diversity.