The genotypic ratio can be find out of a dominant phenotype by test cross or by simply Punnett square.

Phenotypic ratio Tall $:$ Dwarf

Genotypic ratio $TT : Tt: tt$

$1: 2: 1$

A Punnett square used to understand to typical monhybrid cross conducted by Mendal between true-breeding tall plants and true-breeding dwarf plants

Crossing of $F_{1}$ (dominant phenotype) to any one of the parent called back cross and when phenotype of crossing parent is recessive than this is called test cross. The progeny of such cross can easily be analysed to predict the genotype of test organism