Biology 30 Genetics Practice Test

In a dihybrid cross AaBb x AaBb, classic phenotypic ratio?

1:1:1:1 9:3:3:1 3:1 2:2:2:2

Correct Answer:

9:3:3:1

Explanation:

Two genes with independent assortment and complete dominance produce a dihybrid phenotypic pattern. For each gene, Aa x Aa gives a 3:1 phenotypic split (dominant to recessive). Because the two genes assort independently, multiply the probabilities for combined phenotypes. - Dominant for both traits: 3/4 × 3/4 = 9/16 - Dominant for one trait, recessive for the other: 3/4 × 1/4 = 3/16 (two such cases) - Recessive for both traits: 1/4 × 1/4 = 1/16 Putting these together gives a 9:3:3:1 phenotypic ratio. This is the classic result for a dihybrid cross under independent assortment and complete dominance.

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