To test this hypothesis, Mendel selfed the F 2 plants. If his law was correct he could predict what the results would be. And indeed, the results occurred has he expected. From these results we can now confirm the genotype of the F 2 individuals. Remember, the first cross is between two pure line parents to produce an F 1 heterozygote.
Monohybrids are good for describing the relationship between alleles.
When an allele is homozygous it will show its phenotype. It is the phenotype of the heterozygote which permits us to determine the relationship of the alleles.