Background: The Coefficient of Parentage (COP) between two individuals is the expected inbreeding of their offspring. Originally exploited by animal breeders, is now a routine calculation among plant breeders as part of crop improvement programs.
Results: To calculate the COP between two individuals, one selects an allele at random from the same loci of each individual and calculate the probability that they are identical by descent. Nevertheless, to calculate the COP between two strains, we first need to select an individual at random from each strain to perform the calculation. The sampling process adds some independence that simplifies the calculation in the case of strains. We use this fact to provide a simple methodology to calculate the correct coefficient of parentage between strains.
Conclusions: The COP between strains requires a different calculation than the used to calculate the COP between individuals. Failure to do so may result in an overestimation of the amount of inbreeding.