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Typical crossing experiments in Xiphophorus species. X+ and X− represent the presence and absence of the Xmrk gene, respectively. R+ and R− represent the presence and absence of the R gene. F1 hybrids (X+X−, R+R−) express benign melanoma, but they are shown with that of the normal type in this figure. One quarter of backcross offspring will develop melanoma. Modified from Schartl (2008).
