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.