Evaluating the potential roles of the <i>Gray</i> and <i>Extension</i> loci in the coat coloration of Thoroughbred racing horses

Abstract

Horses have substantial variation in coat color, and the genetic loci responsible for the coat color variations have been well investigated. It has been believed that some color variations should follow a single-locus Mendelian law. Examples include the Gray locus that causes the gray phenotype and the Extension locus that specifies the chestnut phenotype. We reevaluated the roles of the Gray and Extension loci by using a large number of mating records of Thoroughbred racing horses. We showed that the data indeed fits the Mendelian law extremely well for the two loci. Furthermore, we demonstrated that the Extension and Agouti loci might have an additional role in determining the degree of melanin that should distinguish bay, dark bay, and brown.

Discriminant analysis of colour measurements reveals allele dosage effect of ASIP/MC1R in bay horses

“…For this locus the authors could not find a difference in colour shade between A/a and A/A. Also Sakamoto et al (2017), who studied segregation ratios by means of pedigree analysis, treated the Extension and Agouti loci separately. For the Agouti locus the authors compared the expected number with the observed number of Our data generally confirm these findings, whereas only the combination of the genotypes A/a with E/E resulted in a statistically confirmed distinct cluster.…”

“…In this study we aimed to characterize and examine single loci effects (ASIP and MC1R) on coat colour variability in bay and black horses. Although ASIP and MC1R and their interaction have been studied (Rieder et al 2001;Sponenberg 2009;Sakamoto et al 2017), the reasons for bay coat colour variability ranging from light bay to seal brown are not fully resolved. This led Sponenberg (2009) postulate a dosage effect and the existence of multifactorial genetic control.…”

“…Whereas A/a and A/A horses did not differ in shades, a significant difference between bay E/e heterozygous and dark bay E/E homozygous horses was found. In a recent pedigree analysis, which was based upon the breeding records and coat colour classifications of 153 778 Thoroughbred horses, Sakamoto et al (2017) analysed the segregation ratios of offspring in relation to different mating combinations. From the segregation ratios Sakamoto et al (2017) concluded that the allele status of ASIP and MC1R affects the shading of bay colour.…”

“…Regarding the phenotypic variability of bay/ brown horses, researchers had to deal with the problem of colour definition, which conventionally is based upon visual inspection and classification. For example Rieder et al (2001) distinguished between the categories bay and dark bay, Sakamoto et al (2017) differentiated bay, dark bay, and brown. In this study we aimed to objectively describe and quantify coat colour variation in bay horses using a spectrophotometer in order to characterize equine coat colour shades according to standardized international procedures as defined by the Commission Internationale de l'Eclairage (CIE) and the L*a*b* colour system.…”

Synergy betweenMC1RandASIPfor coat color in horses (Equus caballus)1

“…Although a statistically significant association between MC1R and coat color has been found, this was based on a limited number of horses (n = 120) and only three colors were investigated (Rieder et al, 2001). Another statistical analysis of coat colors based on 153,778 horses generated a hypothesis that MC1R genotypes have a direct effect on eumelanin levels, but indicated that other genes are involved in this process (Sakamoto et al, 2017). While the genotype distribution of MC1R has been studied, the distribution of ASIP genotypes in horses with different coat colors remains unclear and how the combination of the two important genes correlate with the coat color has not been studied.…”