To fill the Punnett Square we must place the alleles from one of the parents (Bb) horizontally oriented on the top of a 3x3 diagram, and the alleles from the other parent (bb) vertically oriented on the left border of the diagram. After that, we must cross each one of the alleles from one parent with the alleles of the other, positioning the result of those crossings (Bb or bb) in the blank space where those alleles meet, as follows:
Therefore, 2/4 (or 1/2) of the offspring will be heterozygous (Bb) and 2/4 (or 1/2) of the offspring will be recessive homozygous (bb). The genotype ratio is 1:1 (one heterozygous for each recessive homozygous. The phenotype ratio is 1:1 (one organism with a dominant phenotype - Bb - for each organism with a recessive phenotype - bb).
