bone marrow contains stem cells that spawn red blood cells, white blood cells and platelets. In a transplant, ALL the native bone marrow stems cells are first killed and THEN the donor bone marrow stem cells are introduced.
Most DNA tests are done with buccal swabs, and the DNA thus obtained originates from cells that are shed from the inner linings of the mouth and from white blood cells, thus it is no surprise at all that folks who’ve had bone marrow transplants will show their DNA as belonging to the donor ...
I have a friend who had a bone marrow transplant as a leukemia treatment. He was cured but developed a peanut allergy from the donor.