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Mendel’s experiment with tall and dwarf peas, therefore, showed both homozygous and heterozygous zygotes. In pure tall variety, two factors (alleles) remain identical forming TT combination. Similarly, the pure dwarf variety shows tt combination. The hybrid tall plants of the first filial generation (F1) carry two unlike factors, Tt. Mendel in his experiment clearly established the fact that the recessive characters are not expressed in heterozygous condition, but will be expressed in recessive homozygotes (dwarf pea). Ina cross between true-breed contrasting varieties, we can classify F2 progeny (grandchildren grogeny) in two ways, either according to their appearance (phenotype) or according to their genetic composition (genotype). The phenotype classifies thiee individuals as tall out of four, whereas genotype identifies two hybrids (Tt) among the talls. Genotypically the F2 generation is composed of 1 TT, 2 Tt and 1 tt. The terms phenotype and genotype therefore refer to the visible expression of the genes (or Mendelian factors) and the actual genetic constitution of an individual respectively. Mendel discovered the important fact, that in the F1 hybrids, the parental characteristics (or factors or alleles) were not lost or mixed to produce the intermediates, rather the factors or alleles remain segregated. This non-mixing of alleles in the hybrid and their subsequent segregation (equal proportion is maintained in the gamete) is principle of Segregation of Mendel's first law.