The transmission of traits is independent of the presence of other traits.
In Mendel's experiments some peas had smooth yellow seeds, others had wrinkled green seeds. "Smooth" and "yellow" are dominant traits, "wrinkled" and "green" are recessive. Mendel's experiments showed that inheritance of traits is not influenced by the presence of others:
Let A denote smooth, B yellow; thus, the smooth yellow parent (P) generation is AABB. Likewise, let a denote wrinkled and b green, so that the wrinkled green parent (P) generation is aabb.
Because traits cannot influence each other (the A trait cannot extinguish the B trait etc.), the combination of AABB with aabb produces a first (F1) generation AaBb. It contains the dominant traits A and B and therefore has only smooth yellow seeds. Maintaining the principle of independence of traits, the second (F2) generation will then consist of 16 different combinations:
It is seen that the number of smooth yellow, smooth green, wrinkled yellow and wrinkled green peas appears in the ratio 9:3:3:1. Verification of this ratio through experiment proves that traits are transmitted independently of each other.