10.3 Long term genetic contribution

Why do animals have common ancestors? Because their common ancestor apparently was popular enough to have multiple offspring, which, possibly after some generations, resulted in the birth of both parents. The more popular a breeding animal was in the past, the larger the chance that two potential parents have this ancestor in common. The more animals share that common ancestor, the larger the chance that mating two animals will result in an inbred offspring. In other words, there is a relation between the long term genetic contribution of an animal to the population and the rate of inbreeding in the population. The long term genetic contribution is a measure of the level of relatedness between animals in a population because of a shared common ancestor. To illustrate this concept, consider a male that became very famous because he won an important competition. Many breeders decide to use him as mate for their females. In the next generation it becomes clear that he was a champion for good reason because a number of his sons also perform (much) better than average, so they are also used for breeding relatively often. In the next generation, again some sons of these sons are better than average and are used a lot for breeding. You can imagine that within a number of generations, a very large proportion of the animals will have that first champion male as ancestor. If those animals are mated, they are inbred to that champion.

If you would consider your own pedigree and that of your neighbour, you will find common ancestors if you go back far enough in time. Eventually, everyone is related. The point is that how close you are related will depend on the number of offspring that your common ancestor(s) produced and the number of generations between the common ancestor and the present individuals. The more offspring, the more ‘paths’ there are to the common ancestor, and the larger the proportion of genes you share with that common ancestor. The same principle can be applied to animal breeding, though in general the process of shared genes from common ancestors goes a lot faster because the breeding population is relatively small, and introduction of animals from outside the population usually is not practised. After a number of generations, (almost) all animals are related to that common ancestor. After some more generations the contribution of the common ancestor does not change anymore: (almost) all animals have the same proportion of genes of that ancestor in common.