A monohybrid cross is a cross involving one trait only. For example, plant height.
A cross between a tall plant (true breed) and a dwarf plant (true breed)
=> Tall is dominant whereas dwarf is recessive
=> T - tall, t - dwarf
=> F2 ratio is 3 : 1
Parent phenotypes: Tall X Dwarf
Parent genotypes: TT X tt
Gametes: T T t t
F1 genotypes: Tt Tt Tt Tt
F1 phenotypes: Tall Tall Tall Tall
F1 self-crossed
F1 phenotypes: Tall X Tall
F1 genotypes: Tt X Tt
Gametes: T t T t
F2 genotypes: TT Tt Tt tt
F2 phenotypes: Tall Tall Tall Dwarf
F2 ratio: 3 (Tall) : 1 (Dwarf)