Which observation led Mendel to the idea that traits were controlled by more than a single factor?
The observation that led Mendel to the idea that traits were controlled by more than a single factor was the result of his experiments on pea plants and the study of traits. Specifically, Mendel examined the inheritance of two traits: seed shape and seed color. He observed that when he crossed two pea plants with different traits (e.g., one plant with smooth seeds and another with wrinkled seeds), the offspring did not display an intermediate trait but instead consistently showed one of the two parental traits.
Mendel also conducted further experiments where he crossed the first generation offspring with each other (the F1 generation) and observed the traits of the second generation (the F2 generation). In this process, he found that the traits that were not expressed in the F1 generation (the recessive traits) reappeared in the F2 generation. For example, when Mendel crossed two pea plants from the F1 generation that had smooth seeds, he found that some of the offspring in the F2 generation had wrinkled seeds.
This observation of the reappearance of the recessive traits in the F2 generation led Mendel to propose the existence of factors (later known as genes) responsible for controlling traits. He hypothesized that there must be separate, discrete hereditary units, one from each parent, that determined the expression of specific traits in offspring. This laid the foundation for his theory of inheritance and the concept of dominant and recessive traits.
Thus, it was Mendel's observation of the reemergence of recessive traits in the second generation that led him to propose the idea that traits were controlled by more than a single factor.