Ask Mr. Smarty Plants

Before we get any deeper into this, let us explain that the Lady Bird Johnson Wildflower Center is dedicated to the use, protection and propagation of  plants native not only to North America but to the area in which the plants are being grown. None of the citrus fruits (or most of the other fruits and vegetables you would buy at a grocery store) are native to North America, so they are really all out of our area of expertise. We don't know the answer, but we will find some websites about what you have done and perhaps you can figure it out.

eHow How to Grow a Lemon Tree from Seed

Texas A&M Extension Horticulture Home Fruit Production - Lemons  "While cuttings and air layers will work, trees grown on their own roots are not as well-adapted to Texas soils. Seed can be used, but seedlings are slow to bear. Too, some of the seedlings may not come true-to-type."

TreeHelp.com Grafting Citrus Trees

Wikipedia Grapefruit

Okay, what you have here is a tree planted from the seed of a fruit that may not breed true from the seed, and then a graft a year later. Grafting a tree usually requires two trees, one root stock and one the plant you want to perpetuate. What was the other tree? We read all the websites above, and still have no clue what is the answer to your question.