[Campbell Biology P.353] In the simulated experiment with cosmos plants, 900 offspring were ... | Practice Question

In the simulated experiment with cosmos plants, 900 offspring were produced from a testcross AaBb x aabb . If the genes were unlinked, what would be the expected number for each of the four phenotypic classes?

  • A: 900 for each class
  • B: 225 for each class
  • C: A ratio of 9:3:3:1, so 506, 169, 169, and 56 respectively.
  • D: It cannot be calculated without knowing the observed numbers.

Explanation

The null hypothesis states that the genes are unlinked, which predicts a 1:1:1:1 phenotypic ratio in a testcross. With a total of 900 offspring, the expected number for each of the four classes is 900 / 4 = 225.