Dominant and Recessive Traits
Mendel began his experiments with pure breeding strains. These were strains that had been bred for many generations so their progeny always exhibited the same traits, and these traits were always exactly the same in the parents and offspring. In his experiments, he crossed two pure breeding strains that differed only in one trait. The two original strains in this type of cross are called the parental generation, or P. The resulting progeny are called the first filial generation, or F1. Mendel noticed that the F1 plants displayed only one of the traits from the parents. When Mendel allowed the F1 plants to self fertilize, they produced the second filial generation (F2). In this generation, some of the F2 plants had the same trait as the F1 plants (and hence of one of the P plants), but some had the trait not seen in the F1 generation (but seen in the other P strain).
Mendel proposed that one trait was masking the expression of the other trait in the F1 generation but the masked trait could reappear in the F2. He called the trait that was always expressed dominant and the trait that could be masked recessive.
For example, when Mendel crossed plants that produced yellow seeds with plants that produced green seeds, all the F1 plants produced only yellow seeds. After self crossing the F1 plants, some of the F2 plants produced yellow seeds, and some produced green seeds. Specifically, Mendel found that about three quarters of the F2 plants displayed the dominant trait (yellow seeds) and one quarter showed the recessive trait (green seeds). Another way to phrase this is to say there was a 3:1 ratio of yellow to green seeds in the F2. Mendel found that each of the traits he looked at behaved exactly the same way. All had one variation that was dominant in the F1 generation, and all showed a 3:1 ratio of dominant to recessive expression in the F2 generation.