The probability of long type offspring in a cross between two heterozygous tall pea plants can be calculated using the principles of Mendelian genetics.

Mendelian Genetics
Mendelian genetics is based on the laws of inheritance proposed by Gregor Mendel in the 1860s. These laws describe how traits are passed from parents to offspring and can be used to predict the likelihood of certain traits appearing in the offspring of a cross.

Phenotypic Ratio
The phenotypic ratio of the offspring in a cross is the ratio of the different observable traits that appear in the offspring. In the case of the cross between two heterozygous tall pea plants, the phenotypic ratio is 3 tall : 1 dwarf. This means that for every 4 offspring, 3 will be tall and 1 will be dwarf.

Probability
The probability of a particular trait appearing in the offspring of a cross can be calculated using a Punnett square. A Punnett square is a diagram that shows the possible combinations of alleles from the parents and the probability of each combination.

In the case of the cross between two heterozygous tall pea plants, the Punnett square would look like this:

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

The possible combinations of alleles are TT, Tt, and tt. The probability of each combination is 1/4 or 25%.

Since we are interested in the probability of long type offspring, we need to consider the TT and Tt combinations, which both result in tall offspring. The probability of tall offspring is therefore 3/4 or 75%.

Conclusion
The probability of long type offspring in a cross between two heterozygous tall pea plants is 75%. This can be calculated using the principles of Mendelian genetics and a Punnett square.