In the diagram, the genotypic ratio of the offspring of the cross is

The given diagram represents a genetic cross using a Punnett square. A Punnett square is a tool used in genetics to predict the probability of specific genotypes in the offspring of a cross between two parents with known genotypes.

In this case, the gametes T and t represent the alleles of a gene. One parent has the genotype Tt and the other parent also has the genotype Tt. When these parents are crossed, their offspring will have different combinations of alleles (T and t).

Using the Punnett square, we can determine the genotypic ratio of the offspring. The offspring genotypes are as follows:

1. Quadrant I: TT

2. Quadrant II: Tt

3. Quadrant III: tT

4. Quadrant IV: tt

Notice that Quadrant II (Tt) and Quadrant III (tT) are essentially the same genotype, just written in a different order. Thus, we can combine them to simplify the genotypic ratio. The genotypic ratio of the offspring in this cross is 1 TT : 2 Tt : 1 tt, which is represented as 1:2:1.

Therefore, the correct answer is Option B: 1:2:1.