Final answer:
Mendel used pea plants to illustrate that traits were controlled by discrete units now known as genes. His research provided evidence for dominant and recessive traits and formed the foundation of Mendelian genetics, challenging the blending theory of inheritance.
Step-by-step explanation:
When Johann Gregor Mendel was explaining his concept of units of inheritance to his friend, he used pea plants (Pisum sativum) to demonstrate his findings. Mendel discovered that traits, such as pea color or seed shape, were controlled by discrete units, which he later referred to as genes. These genes exist in pairs and offspring inherit one gene from each parent. Contradicting the blending theory of inheritance at the time, Mendel's experiments showed that traits were not blended but rather the units of inheritance kept their distinctness and could be passed onto the next generation. He established the foundational principles of heredity, observing that some traits were dominant, like the purple flower color, over recessive traits, such as the white flower color, which could reappear in the F2 generation.
Mendel's work with hybridizations of true-breeding pea plants led to the law of segregation, which explained how genes are separated during gamete formation and recombined during fertilization, allowing them to be transmitted as distinct units from one generation to the next. This groundbreaking discovery formed the basis of the classical, or Mendelian, genetics that we study today.