He noticed that all the pea plants produced were tall as it was the dominant character trait.

Introduction:
In his experiments with pea plants, Gregor Mendel observed the inheritance of traits from one generation to another. He performed a cross-breeding experiment between tall and short pea plants to understand the pattern of inheritance.

F1 Progeny:
When Mendel cross-fertilized tall pea plants (TT) with short pea plants (tt), he obtained the first filial generation or F1 progeny. The F1 progeny showed a specific pattern of inheritance.

Dominant Trait:
Mendel observed that all the F1 progeny plants were tall. This means that the trait of tallness was dominant over the trait of shortness. The tall trait in the F1 generation was expressed even when one parent had the short trait.

Explanation:
Mendel's observations in the F1 progeny can be explained based on the principles of inheritance:

1. Law of Dominance: According to this law, when two contrasting traits are crossed, one trait (dominant) is expressed in the F1 generation, while the other trait (recessive) remains hidden. In Mendel's experiment, the tall trait was dominant, and the short trait was recessive.

2. Genotypes of the Parents: The tall pea plants used by Mendel were homozygous dominant (TT) for the tall trait, and the short pea plants were homozygous recessive (tt) for the short trait.

3. Genotype of F1 Progeny: All the F1 progeny plants were heterozygous (Tt) for the tall trait. They received one allele for tallness from the tall parent and one allele for shortness from the short parent.

4. Phenotype of F1 Progeny: The F1 progeny expressed the dominant trait of tallness because the dominant allele (T) suppressed the expression of the recessive allele (t). Therefore, all the F1 plants appeared tall.

Conclusion:
In Mendel's experiment, he observed that when tall pea plants were crossed with short pea plants, the F1 progeny were all tall. This indicated that the trait of tallness was dominant over the trait of shortness. Mendel's observations laid the foundation for understanding the principles of inheritance and the existence of dominant and recessive traits.