Final answer:
Mendel's Law of Segregation is demonstrated over three generations: the P generation, the F1 generation, and the F2 generation, where the 3:1 phenotypic ratio in the F2 generation confirms the segregation of alleles.
Step-by-step explanation:
To demonstrate Mendel's Law of Segregation, three generations are needed: the P generation (parental generation), F1 generation (first filial generation), and F2 generation (second filial generation). Mendel's Law of Segregation becomes evident by observing the 3:1 phenotypic ratio that appears in the F2 generation of a monohybrid cross. The pivotal F2 generation reveals the segregation of alleles into gametes, confirming that each parent contributes one allele for any given trait.
During his experiments, Mendel allowed the P generation to cross-pollinate to produce the F1 generation, which were then allowed to self-pollinate to produce the F2 generation. The F2 generation displayed the segregation of the dominant and recessive traits, proving his law. This demonstrates the basic principle that during gamete formation, the alleles for a trait separate so that each gamete carries only one allele for each trait.
For example, if we cross a plant with purple flowers (PP) with a plant with white flowers (pp), all the F1 generation plants will have purple flowers (Pp). When the F1 plants self-fertilize, the resulting F2 generation will have a phenotypic ratio of 3 plants with purple flowers (PP or Pp) to 1 plant with white flowers (pp).