The Selfish Gene

by Richard Dawkins

Having found the generation distance between A and B via a particular common ancestor, calculate that part of their relatedness for which that ancestor is responsible. To do this, multiply by itself once for each step of the generation distance. If the generation distance is 3, this means calculate or . If the generation distance via a particular ancestor is equal to g steps, the portion of relatedness due to that ancestor is . But this is only part of the relatedness between A and B. If they have more than one common ancestor we have to add on the equivalent figure for each ancestor. It is usually the case that the generation distance is the same for all common ancestors of a pair of individuals. Therefore, having worked out the relatedness between A and B due to any one of the ancestors, all you have to do in practice is to multiply by the number of ancestors. First cousins, for instance, have two common ancestors, and the generation distance via each one is 4. Therefore their relatedness is . If A is B’s great-grandchild, the generation distance is 3 and the number of common ‘ancestors’ is 1 (B himself), so the relatedness is