This concept is illustrated in the figure below, where the genetic contributions of 2 sires across 6 generations is shown by the proportion of color in a box. Each sire was mated to 10 dams who had 10 offspring. So in total there were 100 offspring per generation, of which 50 were male. Out of those 50 males only 5 were selected for breeding. And selection was irrespective of the parents, so good sires have more offspring that were used for breeding than poor sires. The contribution of the original sire to the offspring is indicated by proportion of color in the boxes. The sire in the picture on the left was a very successful sire. His offspring was very popular and two of his sons were selected and used a lot. Again their sons and grandsons were popular and, consequently, the proportion of the original sire in generation 6 is large. This in contrast to the sire in the figure on the right. He was not very successful and none of his sons were selected for breeding. Only in generations 4 and 5 males were selected in this family. Consequently, the genetic contribution of this original sire in generation 6 is very small. Note that once the genetic contribution of the original sires have stabilized in the population it is no longer possible to change the size of the contribution in the population.
