Abstract: A new method, the observation method of dominant traits, which can be used for determining the proportion of characters from F₂ under the Mendelian Inheritance, was discovered. The principle of this method is that people can conclude a general formula:3n1 (n1: the number of dominant characters types in F₂) based on the proportion of characters from F₂ under the Mendelian Inheritance. This method is more convenient and time-saving than common tests in teaching and practice of classic genetics.

Key words: base, determine, character, Mendelian inheritance, the observation method of dominant trait, index