@BillRM,
I'll give this one more attempt today.
Your grandfather didn't just have a rough time in WW2. He saw his friends blown up; he spent months in country, stressed beyond our comprehension that his next step may be on a landmine; or he may take a sniper bullet like three people in his company in the last month, and this heightened level of stress changed him at his core.
When he came home, all he'd have to hear was a car backfire or a peal of thunder, smell a certain humid green smell, and he'd start screaming, grab a kitchen knife, and chase your dad or his mom around. Noises triggered him for the rest of his life. One time, he held your dad at knife point and the cops fortunately made it to your house before your dad was killed.
Even though you dad didn't suffer in a war, his life was forever changed because the trauma that directly affected his dad also affected him. The same storm and abrupt noises that triggered granddad also became triggers for your dad. As he became older, he'd get inexplicably nervous and smoke during storms, long after his dad left home or died. Your dad became irrationally short-tempered and developed a drinking problem. He was plagued by anxiety. He was in and out of marriages because he didn't learn interpersonal skills in his home environment.
These things happen. PTSD doesn't just affect the person with the direct stress. They share it with their families.
I believe the depth of trauma suffered for 350 years by a race of people topped off by laws created just to **** with them that were in place for the next 100 years, topped off by a sneakier subset of mistreatments that continues today left indelible marks of varying depth on everyone who identifies as black in this country.
Can you see how that could be possible?