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| Modern Genetic Analysis Chapter 6. Gene Interaction Gene Interaction Leads to Modified Dihybrid Ratios Figure 6-14. Albinism in reptiles and birds. In each case the phenotype is produced by a recessive allele that determines an inability to produce the dark pigment melanin in skin cells. (The normal allele determines ability to synthesize melanin.) (a) Rattlesnake. In this species the normal dark coloration is due entirely to melanin, so the albino allele results in a completely unpigmented appearance. (b) Penguin. In this species melanin normally makes dorsal feathers black, but the reddish-orange colors in the head feathers and beak are due to another pigment chemically unrelated to melanin. The recessive albino allele results in no melanin, but the reddish parts are unaffected and retain their normal coloration. (Part a, K. H. Switak/NHPA; part b, A.N.T./NHPA.)

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