Final answer:
Gregor Mendel first experimented with the characteristic of flower color in pea plants, performing monohybrid crosses with purple and white flowers to examine trait inheritance.
Step-by-step explanation:
The characteristic of pea plants that Gregor Mendel experimented with first was flower color. He began his experiments by cross-pollinating pea plants with purple and white flowers to study the inheritance of this trait. This was part of Mendel's monohybrid cross, which involved parent plants differing in a single characteristic. Eventually, Mendel reported his findings on seven different characteristics, each with two contrasting traits, in his 1865 publication. These traits included seed color, plant height, seed texture, flower color, pea pod size, pea pod color, and flower position. By doing so, Mendel laid the foundation of modern genetics, revealing how characteristics are transmitted discretely from parents to offspring.