In cosmos plants, purple stem (A) is dominant to green stem (a), and short petal

Question

In cosmos plants, purple stem (A) is dominant to green stem (a), and short petals (B) is dominant to long petals (b). In a simulated cross, AABB plants were crossed with aabb plants to generate F1 dihybrids (AaBb), which were then test crossed (AaBb X aabb). 900 offspring plants were scored for stem color and flower petal length. The hypothesis that the two genes are unlinked predicts the offspring phenotypic ratio will be 1:1:1:1.

Using the ratio of 1:1:1:1, calculate the expected number of each phenotype out of the 900 total offspring.

Explanation / Answer

If the genes are unlinked the expected ratio of offsprings would be 1:1:1:1. So, in the current example, the expected and observed ratio is almost similar. For each phenotype, the expected number would be 900/4=225.

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