Is there any example where knowing the truth about genetic causes of personality traits lead to some outcome benefiting humanity?

This example does not conform to Your specification exactly. There is a genetic disease called phenylketonuria, which, if detected early, can be managed by a strict, lifelong diet, low on aminoacid phenylalanine. This prevents mental retardation (plus health problems like seizures). So, in this particular health condition, you can affect IQ by food, and you can do that in time thanks to genetic screening of newborns. IQ is not exactly a personality trait, although we use intelligence as something to describe a psychology of a person. Still, even if the IQ is under the focus of psychology, phenylketonuria is a medical condition, there are other effects than those on intelligence.

Please, does anybody have a better example than this for Viliam ? I am really curious...