Kiger Mustang Directory

Photo: Sabina Elvst√•l  

Calculate Inbreeding Coefficient

Inbreeding is the result if mating between relatives which leads to a decreased level of genetic variability in a population, meaning a higher level of homozygosity (1). The coefficient of inbreeding (COI) is the probability of inheriting two copies of the same allele from an ancestor that occurs on both sides of the pedigree. Thus, a COI of 10% means that 10% of all of the genes in an animal will be homozygous (2).

This site uses Wright´s equation (3) for the calculation, as this is one of the most applied methods. The formula is based on the principle that the inbreeding of an individual is one half the relationship of its sire and dam. 

NB: Wright´s method does not take into consideration the positions of the common ancestors in the pedigree. Furthermore, it only considers duplicated ancestors if they are common to both sire and dam (4). The individual COIs of Wild Kiger Mustangs are not known, and therefore these individuals are assumed to have a COI of zero in the calculations. Keep in mind that these factors will underestimate your result. 

This site provides breeders with a basic tool for the calculation of COI, which may assist in the decision making of a potential mating between horses listed in this database. 

How can this result be interpreted?

Well, first of all, there is no defined limit of an acceptable level of inbreeding in domestic animal populations. But generally, the following COI levels can be applied for the expected inbreeding effects: 

COI < 5%: Significant detrimental effects are rare.

COI 5-10%: Modest detrimental effects on the offspring.

COI > 10%: Significant effects not just on the quality of the offspring, but there will also be detrimental effects on the breed (2). 

Negative effects of inbreeding in horses have been reported in a number of studies. Even modest inbreeding levels (2-5%) are associated with lower percentages of motile and morphologically normal sperm (5). Increased inbreeding is also associated with decreased conception and foaling rates (6), early abortion frequency (7), and higher incidence of retained placenta (8). Inbreeding is often increased over time (1,9), and during the establishment of a new breed it could therefore be wise to keep inbreeding levels reasonably low, since the characteristics for future generations are set. 

It should be noted that inbreeding may also have positive effects on the overall genetic value of a population (10).

The genetic diversity in the wild Kiger and Riddle Mountain herds have been analyzed periodically. The measured inbreeding related to the subpopulation varied over the years but was on average ≤ 3.4 % between 2003 and 2012 (11).


  1. Andersson. Analysis of inbreeding in the Swedish Gotland pony using pedigree information and microsatellite markers. 2010
  2. Accessed Aug. 2019
  3. Wright. Coefficients of inbreeding and relationship. 1922
  4. Accessed Aug. 2019
  5. van Eldik et al. Possible negative effects of inbreeding on semen quality in Shetland pony stallions. 2006
  6. Cothran et al. Inbreeding and reproductive performance in Standardbred horses. 1984
  7. Klemetsdal et al. Effect of inbreeding on fertility in Norwegian trotter. 1989.
  8. Sevinga et al. Effect of inbreeding on the incidence of retained placenta in Friesian horses. 2004
  9. Maciel et al. Population structure and genealogical analysis of the Brazilian Crioula Horse. 2014
  10. Todd et al. Founder-specific inbreeding depression affects racing performance in Thoroughbred horses. 2018
  11. Accessed Aug. 2019