If your talking the, small waisted V-tapered look, then yes, blacks have an advantage. It has been said that the samll waist was an adaptation to the environment (good for running), but having high calves( (a detriment in bbing)., is good for explosive torque.
Whereas you look at European bodybuilder, specifically the ones German and Nordic ancestry, you see thick waists, upper torso, and big calves, good for strength.
What Sir William siad is quite true.
Now as far as being mixed, that is a mixed bag. Because a black (of biracial heritage) could inherit white traits that are bad for bbing (the wide waist), or good for bbing (the long calves). Levrone is mixed, but if you look at the 1993 O he looked odd in some way shape or fashion, he could put on the beef, but he had decent calves (until his later yrs). But his shape could not equal Flex Wheeler. I see no where in Dillett's physique where his white genes are dominant, he just looks like a big black man.
And speaking of Flex, if you look at his physique, you'll see he is third in a line of competitors with that type of shape. First was Jean Guillame (sp), next was Brian Buchanan (both competed at the same time), then Flex, then Melvin Anthony, next Darian Bond (little know pro bber), and then That Ghanain bber who had posted on here (his name excapes me) . Which leads one to conclude that, that type of physique has its origins in Ghana and nearby countries.