Two pea plants, both with genotype Bb, are crossed. Which set of genotypes fills the four boxes?

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Each parent can send B or b, so the four pairings are B with B, B with b, b with B and b with b, and the mixed pair therefore fills two of the boxes. Listing only three genotypes drops one of those two identical boxes, although the square has four boxes because there are four gamete pairings.