Final answer:
When a pure tall pea plant (TT) is crossed with a pure short plant (tt), the ratio of pure tall plants to short plants in the F2 generation is 1:1, as each has a 25% chance of occurring.
Step-by-step explanation:
In Mendelian genetics, when crossing a pure tall pea plant (homozygous dominant TT) with a pure short pea plant (homozygous recessive tt), the first generation (F1) will consist entirely of heterozygous tall plants (Tt). However, when these F1 plants are allowed to self-pollinate, the second generation (F2) will have four possible combinations of alleles: TT, Tt, tT, and tt. The question asks for the ratio of pure tall plants to short plants in the F2 generation. According to Mendel's law of segregation, the probability of getting tt (short plants) is 25%, and for TT (pure tall) or Tt (tall but not pure), it is 75%. But since the question specifically asks for pure tall plants, we must consider only TT genotypes compared to tt. There is a 25% chance for TT and 25% chance for tt, making the ratio 1:1.
Thus, when we cross a pure tall plant (TT) with a pure short plant (tt), the ratio of pure tall plants to short plants in the F2 generation is 1:1.