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| Modern Genetic Analysis Chapter 6. Gene Interaction Gene Interaction Leads to Modified Dihybrid Ratios Figure 6-11. Analysis of the genes for skin pigment in the corn snake. The wild type (a) has a skin pigmentation pattern made up of a black and an orange pigment. The gene o+ determines an enzyme in the synthetic pathway for orange pigment; when this enzyme is deficient (o/o), no orange pigment is made and the snake is black (b). Another gene, b+, determines an enzyme for black pigment; when this enzyme is deficient (b/b), the snake is orange (c). When both enzymes are deficient, the snake is albino (d). Hence the four homozygous genotypes are (a) o+/o+ ; b+/b+, (b) o/o ; b+/b+, (c) o+/o+ ; b/b, and (d) o/o ; b/b. A cross of (a) × (d) or (b) × (c) would give a dihybrid wild-type F1 and a 9:3:3:1 ratio of the four phenotypes in the F2. (Anthony Griffiths.)

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