Final answer:
One of Mendel's seven observed characteristics in pea plants was flower color, with violet and white as contrasting traits. Mendel's experiments with pea plants provided insights into the inheritance of traits, with yellow seeds being dominant over green in the F₁ generation.
Step-by-step explanation:
One of the seven characteristics that Mendel observed in pea plants was flower color. In his 1865 publication, Mendel outlined the results of his breeding experiments, which included observing seven different heritable traits, each with two contrasting forms. Specifically for flower color, Mendel observed the traits of violet versus white.
Mendel's work laid the foundation for the laws of inheritance. For example, when crossbreeding pea plants with different flower colors, he found consistent patterns in how traits were inherited, which helped him formulate his theories.
Regarding your second question about F₁ offspring resulting from a cross between true-breeding green and yellow seed parents, you would expect yellow seeds in the offspring if yellow seed color is dominant over green. This phenomenon is consistent with Mendel's observations and his Law of Dominance.