In a cross between two individuals who are both heterozygous for Tt, if you want to determine the probability that the offspring will have either the genotype TT or the genotype Tt, you would need to use the rule of .

To determine the probability that the offspring will have either the genotype TT or the genotype Tt in a cross between two individuals who are both heterozygous for Tt, you would need to use the rule of addition.

Here are the steps:

1. First, you need to set up a Punnett square. A Punnett square is a diagram that is used to predict the genotypes of a particular cross or breeding experiment.

2. Label the rows and columns of the Punnett square with the possible gametes (T and t) of each parent.

3. Fill in the squares by combining the gametes of each parent.

4. The Punnett square for this cross would look like this:

   | T | t ---|---|--- T | TT | Tt t | Tt | tt

5. Count the number of each genotype. There is 1 TT, 2 Tt, and 1 tt.

6. The probability of an offspring having either the genotype TT or Tt is the sum of the individual probabilities of each event.

7. The probability of TT is 1/4 and the probability of Tt is 2/4 or 1/2.

8. Using the rule of addition, add these probabilities together: 1/4 + 1/2 = 3/4.

So, the probability that the offspring will have either the genotype TT or the genotype Tt is 3/4 or 75%.