6.7.1: Additive genetic relationship

The additive genetic relationship reflects what proportion of their DNA (alleles) two animals share because they have common ancestor(s). Additive genetic relationships can be calculated from the pedigree. Parents pass half of their alleles on to their offspring so the proportion of alleles that parents and offspring have in common is ½. In other words: the additive genetic relationship between a parent and his/her offspring is ½. The offspring received half of their alleles from the father and half from the mother, so their genome is a mixture of genes originating from both parents. When those offspring have offspring themselves, they again pass on half of the alleles to their offspring. What half of the alleles are passed on to the offspring is a random process (Mendelian sampling). Therefore, the proportion of alleles that a grandparent and his/her grand-offspring have in common is ½ (alleles passed on to their offspring) times ½ (alleles passed on from their offspring to their grandoffspring) is ¼.