I meant to write on this immediately after the recent season finale of Downton Abbey but got distracted, so here it is. Although we do not know whether Mr. Matthew Crawley (heir presumptive of Downton) is dead (my guess: he is), what we do know is that Robert Crawley, Earl of Grantham, does have a new heir, the son of his daughter Lady Mary Crawley and her husband Mr. Matthew Crawley, third cousin to the Earl.
In terms of maximizing Darwinian fitness, the real payoff is reproducing with someone of the same race. As Frank Salter calculates in On Genetic Interests, a white parent would be twice as closely related to (share twice as many genes with) a same-race offspring than if he or she reproduced with a black person. Going below the racial level to the ethnic and distant-cousin level, however, there are still payoffs but they become diminishingly smaller.
Nonetheless, in terms of inclusive fitness, the Earl makes out. Not only will his grandson inherit 1/4 of his DNA (or probably even more, depending on genetic proximity of Matthew Crawley), the grandson will also inherit the Earl’s Y-chromosome (through the paternal Crawley line via Matthew) and the surname attached to it. Although HBD Chick might not like it, if I were the Earl of Grantham, I’d be a happy man!