Horse Genetics 4.1: Inbreeding Calculator, Detailed Instructions and Interpretation

 

The Inbreeding Calculator is provided to breeders as a simple and rapid tool for calculating the inbreeding coefficient of an animal based on its pedigree. It is designed to be helpful in performing virtual breeding trials as an aid in making management decisions. The calculator is based on four generations of ancestors (parents, grandparents, great-grand parents and great-great-grandparents), including 30 ancestors in total, using a simplified version of the formula proposed by Sewell Wright for calculating inbreeding coefficients:

In order to further simplify the process, a number is used to designate a specific animal within the pedigree, rather than the complete name of the animal which can be cumbersome. For the first parental pedigree (Pedigree A with ancestors numbered 1 to 15), the numbers are fixed and the parent (animal #1) can be the father or the mother of the offspring in question. For the second parental pedigree (Pedigree B, with ancestors numbered 16 to 30), the numbers can be changed to match common ancestors found in the first parental pedigree. The relative position of common ancestors between the two parental pedigrees is important for the function of the calculator.  It is important to note that ancestors in common within a parental pedigree are not utilized in the calculation, only ancestors in common between the parental pedigrees. This further simplification may result in an underestimation of the absolute value of the inbreeding coefficient calculated for a given animal, however this should not interfere with conclusions made when comparing pedigrees and potential mating partners when performing virtual crosses.

The Inbreeding Calculator is provided as a tool for breeders and is the result of the vision and implementation of Benoit Bouchard, MsEng, a long-time breeder of Pug dogs, in collaboration with Labgenvet and with the help and consultation of Dr. Guy Labbé, DVM. It must be stated that an inbreeding coefficient is just one tool amongst many others available to breeders to help them in making breeding decisions. Breeders are responsible for having an adequate understanding of inbreeding coefficients and for a judicious use of the calculator.

Calculate the inbreeding coefficient for a given animal

To calculate the inbreeding coefficient for a given animal, the paternal pedigree (Pedigree A, representing ancestors 1 to 15) is compared to the maternal pedigree (Pedigree B, representing ancestors 16 to 30). The relative position of an ancestor in common between the paternal and maternal pedigrees is important for the proper functioning of the calculator. Note that in Pedigree A, positions 2, 4, 6, 8, 10, 12 and 14 are marked by blue rectangles and represent male animals while positions 3, 5, 7, 9, 11, 13 and 15 are marked by pink rectangles and represent female animals. In a similar fashion, in Pedigree B, positions 17, 19, 21, 23, 25, 27 and 29 represent male animals while positions 18, 20, 22, 24, 26, 28 and 30 represent female animals.

Evidently, for ancestors in common between the two pedigrees, male animal numbers from Pedigree A (2, 4, 6, 8, 10, 12 and 14) should only replace male positions in Pedigree B (17, 19, 21, 23, 25, 27 and 29). In a similar fashion, female animal numbers from Pedigree A (3, 5, 7, 9, 11, 13 and 15) should only replace female positions in Pedigree B (18, 20, 22, 24, 26, 28 and 30). The positions of the parents are marked by yellow rectangles. If the father is represented by position 1 in Pedigree A then the mother will be represented by position 16 in Pedigree B. Alternatively, if the mother is represented by position 1 in Pedigree A then the father will be represented by position 16 in Pedigree B.

In this version of the calculator, the inbreeding coefficient is calculated based on common ancestors found between the two parental pedigrees but does not consider common ancestors within parental pedigrees. Remember to reinitialize the calculator between calculations by pressing the Reset button.

Calculate the inbreeding coefficient of a possible offspring resulting from a virtual cross

Your animal is represented as animal 1 in the upper parental pedigree (Pedigree A, with animals 1 to 15) and can be either the potential father or mother. The animal that you are considering as a potential breeding partner is represented by animal 16 in the lower parental pedigree (Pedigree B, with animals 16 to 30). The numbers in the lower parental pedigree are changed to reflect common ancestors with the pedigree of your animal. The relative position of an ancestor in common between the paternal and maternal pedigrees is important for the proper functioning of the calculator. In this version of the calculator, the inbreeding coefficient of the potential offspring resulting from the virtual breeding of the proposed parents is calculated based on common ancestors between the two parental pedigrees; common ancestors within parental pedigrees are not considered. Do not forget to reinitialize the calculator between calculations by pressing the Reset button.

Interpretation of inbreeding coefficients

An elevated inbreeding coefficient for an animal indicates that the undesirable effects of inbreeding (i.e. inbreeding depression) will start to be evident. On the other hand, an elevated inbreeding coefficient will increase the chances that desirable traits associated with the breed in question will be fixed. Thus, the inbreeding coefficient can be viewed as a compromise. The deleterious effects associated with inbreeding start to be seen when the coefficient of inbreeding is higher than 5%, which is just below the value obtained for the offspring of a mating between two cousins (=6.25%). It is advised to maintain a coefficient of inbreeding that is below 10% which should allow a number of desired traits to be fixed without allowing the undesirable effects of inbreeding to become too pronounced. Incestuous crosses resulting in offspring with coefficients of inbreeding above 12.5% should not be performed; these include parent-offspring, brother-sister, grandparent-grandchild and half-brother-half-sister matings. In practice it is recommended to choose crosses that will result in offspring that have reduced coefficients of inbreeding compared to the average of the breed in question. If a number of breeding possibilities are available that will reduce the average inbreeding coefficient within the offspring compared to the breed average, then ideally the breeding that will result in the lowest inbreeding coefficient while still maintaining the desired traits for the breed is recommended.

General recommendations for breeding:

  1. Choose crosses that will reduce the inbreeding coefficient of the resulting offspring relative to the average for the breed (if this is available).
  2. If possible, avoid using an animal with common ancestors within its pedigree, at least not within the 3 to 4 most recent generations.
  3. Avoid incestuous breedings, with inbreeding coefficients of 12.5% and above.
  4. Keep inbreeding coefficients below 10%.
  5. Try to keep inbreeding coefficients below 5%.
  6. Think about sacrificing a bit of “type” for the sake of preserving the genomic health of your breed.