Ronda Rousey's mom just bro'd out with our African American camera guy ... telling him they've got something in common -- their blackness!
Ronda was hanging with her mom and sister out in L.A. on Tuesday when her mom busted out the black fun fact ... not only is Ronda's great grandfather black ... but he's kinda famous!
Turns out, his name is Dr. Alfred Waddell -- and according to Ronda's mom he was "one of the first black physicians in North America."
There's a TON of information on Waddell -- who was known as a major pioneer in the Canadian civil rights movement.
And that explains it ...
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