Probability theory
Generally, the monohybrid cross is used to determine the dominance relationship between two alleles. The cross begins with the parental generation. One parent is homozygous for one allele, and the other parent is homozygous for the other allele. The offspring make up the first filial (F1) generation. Every member of the F1 generation is heterozygous and the phenotype of the F1 generation expresses the dominant trait. Crossing two members of the F1 generation produces the second filial (F2) generation. Probability theory predicts that three quarters of the F2 generation will have the dominant allele's phenotype. And the remaining quarter of the F2s will have the recessive allele's phenotype. This predicted 3:1 phenotypic ratio assumes Mendelian inheritance.