>>18073573The basis of it is that if you need a transplant, you want your organ donor to be as genetically similar to you as possible. The greater the genetic distance between you and your organ donor, the more likely your body is to perceive the organ as a hostile foreign invader and try to kill it with white blood cells.
Usually this means an immediate family member, or someone with ancestors from the same part of the world as you. But with mixed-race children, both parents are genetically distant and that raises the risk of transplant rejection. There's more of an explanation here
>>18073567 