Pedigree-Based Assessment of Genetic Structure and Disease-Associated Variants in Friesian Horses in Brazil.

Abstract: Friesian horses are recognized for severe genetic restriction due to intensive selection. Still, the genetic diversity, founder representation, and prevalence of inherited disorders in the Brazilian Friesian population, which is comprised of fewer than 500 individuals, have not been previously investigated AIMS/OBJECTIVES: characterize the genetic diversity, inbreeding, founder representation using pedigree-based tools, and frequency of known pathogenic, behavioral, and white spotting alleles in Friesian horses registered in Brazil METHODS: Pedigree data from 411 Friesian horses (2003-2024) were analyzed to estimate the mean pedigree-based inbreeding coefficient (F), and other pedigree-based population diversity measures using the R package PurgeR. Genetic testing was performed in 12 horses for pathogenic variants previously implicated in dwarfism and hydrocephalus, as well as depigmentation alleles, and the dopamine receptor D4 (DRD4) polymorphism associated with temperament RESULTS: Pedigree-based estimates (F:16.25 ± 3.8 %, Ne = 32.6, Nae = 10.19) indicate a critical loss of genetic variability (p < 0.0001). Three stallions contributed 36% of all offspring, suggesting a popular sire effect. Carrier frequencies were 33% for the dwarfism variant and 16.7% for the hydrocephalus variant. Depigmentation alleles were present in 25% of horses without visible white markings. The DRD4 "curious" G/G genotype predominated (75%), indicating potential locus selection CONCLUSION: The Brazilian Friesian horse population exhibits low effective population size, high inbreeding, and moderate frequencies of deleterious alleles despite active importation of genetic material. Implementation of genetic testing, sire usage limits, and monitoring of inbreeding coefficients are recommended to maintain population health and breed integrity.