We see what happens when too many "good genes" are passed around-----when people marry "within the family."
This happened with many royal families in the past. This is known as "inbreeding."
Fortunately, it could be said that Princess Diana, actually, "went against the grain." Prince Charles was smart enough to marry someone who is far from their lineage. Both their kids turned out really well.
Eugenics does have its drawbacks.