Final answer:
Gregor Mendel observed that traits in pea plants are inherited as separate units, leading to his conclusion that traits do not blend in offspring. This was foundational to the principles of genetics, including segregation and independent assortment.
Step-by-step explanation:
Gregor Mendel's observation that pea plant traits did not blend in their offspring led him to conclude that: (A) Traits are inherited in separate units. Mendel's work with pea plants revealed that traits are controlled by discrete units that occur in pairs. These units, which we know as genes, are inherited from each parent, with offspring inheriting one allele from each parent. His experiments demonstrated that these hereditary factors are not blended but are passed down as distinguishable units. This observation was crucial for the development of the principle of segregation and independent assortment, as well as the foundation of classical genetics.
In his experiments, Mendel crossed peas with contrasting traits such as violet versus white flowers. The reappearance of traits in the F2 generation demonstrated that traits remain distinct and are not absorbed or blended. Mendel's principle of segregation states that individuals possess two copies of each trait and that each parent transmits one of its two copies to its offspring. Thus, the traits that are passed on are a result of inherited genes on chromosomes during meiosis and fertilization, as supported by the Chromosomal Theory of Inheritance, proposed in 1902.