When Gregor Mendel crossed true-breeding tall plants with true-breeding short plants, why was it impossible to observe segregation? . A. Alleles for height do not segregate in the F2. . . B. Alleles segregate only in the F2 generation. . . C. Alleles segregate best when two tall plants are crossed. . . D. Alleles in the F1 must be Tt to have height variety in the F2.