During meiosis two alleles for each trait separates. All resulting F1 generation plants have the genotype Yy and have yellow seeds because yellow seeds are dominant to green seeds. These are called hybrids.
Mono hybrid cross
Mendel allowed the Yy plants to self-fertilize. This is called mono hybrid cross. These plants make 2 types of gametes, males and females. These contain either the y or Y chromosome. This random fertilization can result in YY, Yy, Yy, or yy genotypes.
Genotype ratio is 1:2:1
Phenotype ratio is 3:1