Platyfish with spots have an x-linked gene that produces the spots. all platyfish have an autosomal repressor that inhibits the expression of the spot-producing gene. the closely related swordtail lacks both the spot-producing gene and the repressor. in backcrosses between these two species, some hybrids receive the spot-producing gene, but not the repressor. these individuals can develop malignant tumors, because the expression of the spot-producing gene is not properly regulated. this appears to be an example of: