We got two annoying tropes in one with Dr. McCoy: First, we have the dark-haired girl that according to fiction politics has to die in order to be portrayed as a victim compared to the blonde girl who according to fiction politics isn't allowed to die. Then we have the equally disgusting conceit that any doctor would be mad enough to link the two emotionally or think that he might be somehow absolved for the death of the first girl by saving the second, let alone his own daughter. Dead is dead. No doctor in the world would be so deranged as to feel that the death of one person somehow is made less of a tragedy because he could save another person. There was also no reason whatsoever to make the girl who died a dark-haired girl, it might as well have been a blonde girl, especially since the doctor is blonde himself. But as usual, writers every now and then feel the uncontrollable desire to portray dark-haired women as victims and blonde ones as victors in life. A mental disorder for which a successful therapy or medication has not been developed yet.