In experiments with garden peas carried out between 1856 and 1868, Gregor Mendel, an Austrian monk, crossed varieties of peas which among them had alternative values of seven characteristics or traits, terming the original parental generation the "P" generation. As each of Mendel's seven crosses was concerned with only a single characteristic each can be called a monohybrid cross. For example, he crossed peas that had green pods with peas that had yellow pods. All the first ("F1") generation peas from such a cross always had green pods. In fact, all the F1 generation peas from each of the seven crossed characters always took on only one of the two contrasting parental forms and always took on the same parental form. Thus, seven forms were always expressed by the F1 generation peas while seven forms were never expressed. Mendel surmised that one form of each character took precedence over the other form. He termed the forms, such as the green pods, that appeared in the F1 generation of peas, dominant, and the forms, such as the yellow pods, that did not appear in the F1 generation, recessive. Mendel then crossed the members of the seven different types of F1 generation of peas with the other F1 peas that shared the same characters. In the second (F2) generation of peas that resulted, three quarters of the peas showed the dominant forms of their F1 generation parents while one quarter showed the recessive forms that had not been expressed since the original P generation. For example, crossing the peas with green pods from the F1 generation resulted in a F2 generation in which three quarters had green pods and one quarter had yellow pods.
