Definition
A generation, in biological terms, is the interval between the birth of an individual and the birth of that individual’s offspring. In horse breeding, it refers to one step in a pedigree — moving from parent to offspring. A dam and stallion constitute one generation above their foal; the foal’s grandparents are two generations back.
Generation Interval in Horses
The generation interval for horses is the average age of parents at the time they produce offspring used for breeding. In horses, this typically ranges from eight to twelve years, substantially longer than in cattle, pigs, or dogs. The long generation interval means that selective breeding for new traits — or selection against a disease heritable trait variant — takes decades to show measurable population-level change. A trait that requires three generations to fix at high frequency in a breed will take twenty-five to thirty-six years to accomplish, compared to six to ten years in a species with a shorter interval.
Generations in Pedigree Analysis
Pedigree records trace generations back to establish inbreeding coefficients and identify influential ancestors. A pedigree showing three to five generations is standard for breed registration and most breeding decisions. When analyzing the breed-level allele diversity of a breed, counting the number of generations back to foundation animals indicates how concentrated the ancestry is. Heavily inbred animals may share the same ancestors multiple times across different branches of their pedigree, and the number of generations separating the common ancestor from the current individual determines how much genetic material is likely still shared.
Generation Turnover and Breed Change
Breed type shifts over generations as breeders select for changing performance goals. The modern Thoroughbred has changed measurably since early eighteenth-century foundation stock due to consistent selection pressure across roughly twenty generations. In warmblood breeds, rapid improvement in sport performance over the last forty years — approximately four to five generations — reflects intense selection combined with objective performance testing of young approved sires before breeding approval.
Further reading: Generation on Wikipedia; Thoroughbred at Britannica (a breed whose documented generation history spans 300 years of selective pressure).