This is interesting, but all so confusing. How is it that we are not descended from all who came before? Perhaps some more than others, depending on how many cousins and cousins once removed married each other.
Check out the earlier diagram. Also some family lines are thin, for example, a friend of mine has no siblings; he has no first cousins on his mom’s side (she had no siblings); his mother had no first cousins on her dad’s side (he had no siblings that survived). Meanwhile, parents who have three kids have a good shot at passing on all their chromosomes.