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

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