These are the results of me looking at the source code of IndustrialCraft 2 for determining the offspring of crossbred plants.
The results of cross breeding are entirely dependent on the attributes of the parent plants. A parent plant is any crop that is adjacent to the crossbreeding crop block. If you have a specific target plant in mind, you must choose parent plants with similar statistics, attributes, and tiers.
The similarities affect the score of the target plant by the chart on the right. The parent plant contributes 500 score to an offspring that is a clone of itself. A higher score is a higher chance of the offspring becoming the desired plant, weighed against the scores of all the other target plants.
To find parent crops that, when crossbred, would create the desired target crop, refer to the chart on the bottom. Do your best to find parents that are similar to your desired plant, and click the "add" button to see the score distribution of the target plants.
Although the scores that are shown are a good reference for how you should breed plants, they are not exact probabilities of getting the target plants. The algorithm that is coded does not weigh the scores directly, but instead has an additional layer of probability. This is to encourage more experimentation and discourage breeding mathematically optimal plants. The bottom line is, I strongly encourage you use differing parent plants and experiment. Experience is the best guide in this case.
|Statistics||Each similar statistic contributes up to +2 to score. Dissimilar statistics penalizes -1.|
|Attributes||Each matching attribute contributes +5 to score.|
|Tiers||Penalizes score if tiers are off by more than 1.|