To overly simplify- if there was a 65% white 35% black child pretty much only other 65% white and 35% black could donate (marrow? Plasma? I forget which aspect tbh). If the parents aren't close to being 65% white and 35% black they are highly likely to not be a match. "Our research demonstrates that many people will never find a matched volunteer donor from any registry because of their racial and ethnic background," Basically siblings are best bet because its same mix ratio and beyond that good luck. Just read articles on why its hard to find donors. They may be more dodgy with wording but its obvious what is actually occurring