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

This study analyzed pedigrees and limited genetic tests in Brazil’s small Friesian horse population and found high inbreeding, a few stallions dominating reproduction, and moderate frequencies of harmful recessive variants. The authors recommend routine genetic testing, restricting overuse of popular sires, and tracking inbreeding to protect the breed’s health.

Background and Rationale

• Friesian horses are globally known to have restricted genetic diversity due to historical and ongoing intensive selection.
• Brazil’s registered Friesian population is small (fewer than 500 horses), and before this work there had been no systematic assessment of its genetic structure or inherited disease burden.
• Understanding inbreeding, founder representation, and deleterious variant frequencies is critical to prevent further loss of diversity and to manage health risks in a numerically limited population.

Objectives

• Quantify pedigree-based genetic diversity and inbreeding in Brazilian Friesians.
• Characterize founder and ancestor contributions to identify unequal reproductive use (popular sire effect).
• Estimate the frequency of known Friesian-associated pathogenic variants (dwarfism and hydrocephalus), depigmentation alleles related to white spotting, and a temperament-associated DRD4 polymorphism.

Methods

• Pedigrees from 411 Friesian horses (2003–2024) were analyzed using the R package PurgeR to compute:
  • Mean pedigree-based inbreeding coefficient (Fped).
  • Effective population size (Ne), reflecting the number of breeding individuals that contribute to genetic diversity.
  • Effective number of ancestors (Nae), indicating how many ancestors explain the current gene pool when accounting for bottlenecks and unequal contributions.
• Genetic testing was performed in 12 horses for:
  • Pathogenic variants associated with Friesian dwarfism and hydrocephalus.
  • Depigmentation (white spotting) alleles.
  • A dopamine receptor gene polymorphism (DRD4) linked in prior studies to “curious”/exploratory temperament.
• Note on inference: Molecular results derive from a small sample (n = 12), so frequency estimates carry wide uncertainty and should be considered preliminary indicators.

Key Results

• Pedigree diversity and inbreeding:
  • Mean Fped = 16.25% ± 3.8%, indicating high average autozygosity across the population.
  • Effective population size Ne = 32.6, a critically low value for short-term genetic health.
  • Effective number of ancestors Nae = 10.19, reflecting a narrow set of influential ancestors.
  • Overall signals indicate a critical loss of genetic variability (p < 0.0001).
• Founder/ancestor representation:
  • Three stallions sired 36% of all offspring, a pronounced popular sire effect that accelerates coancestry and inbreeding.
• Variant frequencies in the genotyped subset (n = 12):
  • Dwarfism variant carriers: 33%.
  • Hydrocephalus variant carriers: 16.7%.
  • Depigmentation (white spotting) alleles: 25% in horses with no visible white, indicating cryptic carriers or low penetrance.
  • DRD4 “curious” G/G genotype: 75%, suggesting potential selection at this locus.

Interpretation and Context

• The mean inbreeding coefficient of 16.25% exceeds that expected from a single half-sibling mating (12.5%), implying multiple generations of related matings and a narrow breeding base.
• Ne = 32.6 is below commonly cited short-term conservation thresholds (~50) and far below values recommended for long-term adaptability (~500), consistent with rapid drift and accumulation of homozygosity.
• The expected rate of inbreeding per generation is approximately ΔF ≈ 1/(2Ne) ≈ 1.5%, which is high for a managed domestic population.
• Nae ≈ 10 indicates that, despite many registered founders overall, the current gene pool is effectively traced to about ten disproportionately influential ancestors, consistent with the observed popular sire effect.
• The concentration of offspring from three stallions (36%) intensifies coancestry, shrinking Ne and raising the risk of recessive disease expression in subsequent generations.
• Health variant frequencies in the tested subset are concerning for a numerically small population:
  • If true population carrier frequency for dwarfism were near 33%, random matings would yield an estimated ~2.7% affected foals (0.25 × 0.33²), underscoring the value of pre-breeding testing. However, the 95% confidence interval from n = 12 is wide (roughly ~10%–66% for carriers), so population estimates require larger samples.
  • For hydrocephalus, a 16.7% carrier rate would imply ~0.7% affected under random mating, again with wide uncertainty from small n.
• Detection of depigmentation alleles in phenotypically solid-black horses reveals cryptic color genetics; these alleles can sporadically produce disallowed white markings, relevant for breed standards and selection policies.
• The predominance of the DRD4 G/G genotype (75%) may reflect inadvertent selection for temperament-associated alleles, which can co-segregate with other genomic regions and further reduce diversity; behavioral associations are probabilistic rather than deterministic.
• Despite importation of genetic material, the effective diversity remains low, suggesting that imported lines are themselves closely related within the global Friesian founder pool or are being used in ways that do not broaden contributions (e.g., repeated use of a few popular imported sires).

Implications for Breeding Management

• Implement routine pre-breeding genetic testing:
  • Screen all breeding stock for Friesian dwarfism and hydrocephalus variants; avoid carrier × carrier matings using mate allocation tools.
  • Include depigmentation allele testing to manage the risk of disallowed white markings.
• Limit popular sire usage:
  • Set caps on the number of registered offspring per stallion per season and over a lifetime to curb coancestry growth.
  • Promote balanced use of multiple unrelated or underrepresented stallions.
• Monitor and manage inbreeding:
  • Track Fped for all proposed matings and prioritize pairings that minimize progeny F.
  • Adopt optimum contribution selection to control the rise of mean kinship while meeting selection goals.
• Targeted importation and selection:
  • When importing semen or animals, prioritize lineages that demonstrably reduce mean kinship rather than repeating already overrepresented lines.
• Data infrastructure:
  • Maintain complete, accurate pedigrees and share genotype results within the registry to enable population-wide mate planning.

Limitations

• Genetic testing was limited to 12 horses, yielding imprecise allele frequency estimates; larger, representative sampling is needed to confirm true population frequencies.
• Pedigree-based metrics can underestimate or overestimate genomic inbreeding if pedigrees are shallow or contain errors; genomic measures (e.g., runs of homozygosity) would strengthen inference.
• The study focuses on registered horses; unregistered or recently imported individuals not in the dataset may alter estimates if included.

Future Directions

• Expand molecular screening to a larger, random sample of the population to refine carrier frequency estimates and inform mate allocation.
• Incorporate genome-wide SNP data to estimate genomic inbreeding (FROH), effective population size trajectories, and to detect selection signatures.
• Quantify additional pedigree metrics (e.g., effective number of founders and founder genomes) to complement Nae and guide diversity-oriented breeding goals.
• Evaluate trends over time (2003–2024) to document whether management changes reduce Fped growth and increase Ne.

Bottom Line

• Brazil’s Friesian population shows high inbreeding, a narrow ancestor base, and nontrivial frequencies of deleterious recessive variants. Immediate steps—routine genetic testing, limiting popular sires, and active inbreeding management—are warranted to safeguard health and maintain breed integrity.