Calculating Inbreeding Through Path Analysis
How dog breeders calculate their line’s coefficient of inbreeding.
D. Caroline Coile, Ph.D.
One way to measure inbreeding is by a technique called path analysis.
- Re-draw your dog’s pedigree, but instead of writing it out in the traditional manner, every time the same name appears on both the sire’s side and the dam’s side, write the name only once.
- Draw a path from your contemplated litter to the litter’s sire, back through each of the sire’s ancestors to that common ancestor.
- Do the same through your dog’s dam. Now you should have a circular path through several generations that goes via both the sire and dam’s sides of your dog’s pedigree.
- Count the number of steps in the circular pathway from your dog to the common ancestor, and subtract 1 from that number.
- The contribution of each step in the pathway is (1/2)n, where n is the number you got in step 4. For example, for three steps: (1/2)2 is 0.25.
- Many dogs will have more than one ancestor common to sire and dam, so repeat these steps for each common ancestor.
- Add the contributions of each path together. For example, [if the sire and dam had three common ancestors): (1/2)3 + (1/2)4 + (1/2)6 ) to obtain the F value, also called the coefficient of inbreeding (COI). In this example, F = 0.125 + 0.0625 + 0.015625 = 0.203125, or a COI of about 20 percent.
- COI increases if the common ancestor is itself inbred. The adjustment to the calculation is made by multiplying the (1/2)n result by (1 + Fa), where Fa is the COI of the common ancestor. If the common ancestor had a COI of 0.0125, then you would multiply the (1/2)n value by 1.0125.
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