08/09/2025
There has been a lot of discussion recently about inbreeding and linebreeding, so I want to take a moment to discuss the two.
In order to understand inbreeding and linebreeding, you must first understand what an inbreeding coefficient is. The coefficient of inbreeding (CoI) is the probability that two alleles at any locus in an animal are identical, having descended from a common ancestor of the two parents. This number is an estimation of how in**ed an animal is.
The animal registries that do research on inbreeding have used 12.5% CoI as the cutoff between Inbreeding (12.5% and above) and linebreeding (below 12.5%). Therefore, this is considered the acceptable definition.
The CoI is calculated by assuming that an animal used for breeding is passing on half of its genes from either of that animal’s parents. This is just an average, and animals may pass on more genes from one of their parent ls than the other since meiosis randomly splits up the genes for each chromosome. But if we take an example, an animal inherits half of its genetic material from dad and half from mom. It will then pass half that genetic material on to its offspring. So breeding a full brother to a full sister is 0.5 x 0.5 = 0.25 CoI (50% x 50% = 25%).
There are quite a good number of tools now to get a more accurate CoI available through the companies that do genetic testing. Several of the labs can give you an inbreeding coefficient range for a prospective foal if they have both sets of parent DNA. They can also give you an inbreeding coefficient for your own horse. However, using averages can give you a rough estimate of the CoI without having exact genetics.
There is a diagram floating around with concentric circles that I've noticed a lot of people using which is incorrect. It shows full brother and full sister sharing 100% of their DNA. This is not true. Only identical twins share identical DNA. Full brothers and sisters cannot share all of their DNA, simply by virtue of the fact that one is male and one is female and therefore they inherited different s*x chromosomes at the very least. I'll add it in the comments so you know to avoid this diagram since it is wrong.
You can use the diagram I created with this post to calculate your inbreeding coefficients of your own stock. The CoI is additive, so if you had a foal that was the offspring of first cousins and also third cousins, you would add the two numbers together: 6.25+.39 = 6.64%
This is meant as an educational post and not to draw attention to anyone's program. It is meant as a tool to help you make your own breeding decisions.
Here is a tool you can use by entering the pedigree of your horse or of a prospective foal to determine the inbreeding coefficient. This is especially helpful when you have two or more of the same ancestor in the pedigree, because it will add the inbreeding coefficients for you. https://www.nabssar.org/coi_calculator/index.html
