Mendel obtained the recessive character in $F_{2}$ by self pollinating the $F_{1}$-plants.

Mendel cross-pollinated a pure tall pea plant ($100-120\, cm$ hight) and a pure dwarf pea plant. (only $22$ to $44\, cm$ hight). He called them parental generation, expressed now-a-days by symbol $P$.

This hybridization popularly called as monohybrid cross

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This three generations of pea plants after crossing a pure tall plant with a dwarf one. The plants of $F_{1}$-generation are all tall, of $F_{2}$ generation three tall and one dwarf. One third of the tall plants are pure, while the remaining behave as hybrids

$F_{1}$-generation Seeds collected from the parental generation called first filial generation or $F_{1}$-generation

$F_{2}$-generation $F_{1}$-plants pollinated among them self (self breeding or inbreeding) and seed produced by $F_{1}$-plants called $F_{2}$-generation. They were in ratio $3: 1$ (three tall and one dwarf).

$F_{3}$-generation Mendel allowed $F_{2}$-plant to form seed by self-pollination called $F_{3}$-generation. Mendel observed that tall and dwarf plant behave differently

$(i)$ Dwarf plant produced dwarf plant on self-pollinated

$(ii)$ In tall plants one third plants breed true so they were pure

$(iii)$ Other two third plant behave like parents and give tall to dwarf plants $3: 1$ indicate that their parents have dwarf genes also