Q.In order to obtain the F1 generation Mendel pollinated a pure-breeding tall plant with a pure breeding dwarf plant. But for getting the F2 generation, he simply self-pollinated the tall F1 plants. Why?

Mendel’s choice to self-pollinate the F1 plants, rather than cross them again, was deliberate and grounded in the logic of his experimental design. The F1 generation from a pure tall × pure dwarf cross were all tall, showing that tallness was dominant. If he had crossed these F1 plants with a dwarf plant (a test cross), he would have gotten a 1:1 ratio of tall to dwarf — useful for confirming dominance, but not for revealing the full pattern of inheritance. …

Mendel’s choice to self-pollinate the F1 plants, rather than cross them again with a pure parent, was not an accident — it was a deliberate and elegant experimental design. To understand why, you need to step into his shoes. He had just crossed a pure tall plant (TT) with a pure dwarf plant (tt). The F1 offspring were all tall, but Mendel suspected they were not pure tall — they carried a hidden factor for dwarfness. The question now was: how to test that suspicion?

If he had crossed the F1 tall plants with a pure tall parent (a backcross), the offspring would all be tall, and the hidden dwarf factor would never show itself. If he crossed them with a pure dwarf parent (a test cross), he would get a 1:1 ratio of tall to dwarf — but that would tell him only about the F1’s genetic makeup, not about how traits segregate naturally when no external pollen is introduced. Mendel wanted to see what happened when the F1 plants reproduced on their own, without any interference.

Self-pollination was the perfect tool. Pea plants are naturally self-fertilising — each flower contains both male and female parts. By simply letting the F1 tall plants pollinate themselves, Mendel ensured that the only genetic material involved came from the F1 plants themselves. No foreign pollen, no artificial mixing. This allowed the two alleles (T and t) in each F1 plant to separate during gamete formation and then recombine randomly during fertilisation. The result was the famous 3:1 ratio of tall to dwarf plants in the F2 generation.

Compare the three possible next steps side by side: crossing F1 back to the tall parent would mask the hidden recessive factor completely (all offspring tall); test-crossing F1 to the dwarf parent would reveal the 1:1 ratio but requires a second stock plant and a fresh, deliberate cross each time; self-pollination needs no seco …