Inbreeding is a result of mating related animals. If we know the relationship between animals, we would be able to predict and, to some extent, control inbreeding level in the next generation. If we know the pedigree of the animals it is possible to calculate the level of relatedness between animals, and thus the level of inbreeding of an individual animal. In the following part of the chapter we will look into more detail at how to perform these calculations.
Two animals are related when they have one (or more) ancestors in common. For example, you are related to your cousin because you have the same grandparents, they are your common ancestors. Because you have common ancestors, you and your cousin will have part of your alleles in common. The key-issue is that related individuals have alleles in common.
Thus:
Related individuals have similar alleles
In figure 6 you see two simple pedigrees. In pedigree 1, animals A and B are parents of animals C and D. In other words: A and B are common ancestors of C and D, who thus are full brother and sister. Animals C and D are mated and have offspring E. Because C and D are related, E is inbred. In pedigree 2, animals F and G are parents of H and I. Animals H and I are mated to non-related animals and have offspring J and K. Those are mated and have offspring L. Animals J and K are related because they have ancestors F and G in common. So animal L is inbred, but less than animal E, because J and K are less related than C and D.
The more generations to the common ancestor, the less related two animals are. The less two mated animals are related, the smaller the probability that the same allele is passed on to their offspring, so the less the offspring is inbred.
Thus
Inbreeding level decreases with decreased relatedness between parents