The 9:3:3:1 Ratio
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Self-crossing a dihybrid F1 heterozygote produces an F2 generation with four phenotype classes in a 9:3:3:1 ratio, when both traits assort independently.
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A YyRr self-cross yields roughly nine yellow-round, three yellow-wrinkled, three green-round, and one green-wrinkled offspring per sixteen.
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This exact ratio holds only under independent assortment — Bio 1067's linked loci produce a different, skewed proportion instead.