Mendel's success stemmed from his scientific rigor:
1. Choice of Organism: Pea plants were ideal because they were easy to cultivate, had a short generation time, produced many offspring, and allowed for controlled cross-pollination.
2. Focus on Discrete Traits: He studied distinct, easily observable traits that had two contrasting forms (e.g., tall vs. dwarf, yellow vs. green seeds, round vs. wrinkled seeds).
3. Use of Pure-Breeding Lines: He started his experiments with "pure-breeding" (true-breeding) varieties, meaning that when self-pollinated, they consistently produced offspring identical to the parent for that trait over many generations. This ensured a known genetic starting point.
4. Quantitative Analysis: Crucially, Mendel counted and analyzed thousands of offspring, allowing him to identify consistent numerical ratios, which were key to his deductions.

1. The Law of Segregation (Monohybrid Crosses)
   This law explains how a single heritable trait is passed from one generation to the next. Mendel performed monohybrid crosses, involving only one pair of contrasting traits.
2. Experimental Setup and Observation:
3. Mendel crossed pure-breeding tall pea plants with pure-breeding short pea plants. This was the Parental (P) generation.
4. The first filial (F1) generation consisted entirely of tall plants. The "short" trait seemed to have disappeared.
5. He then allowed the F1 tall plants to self-pollinate (or crossed F1 plants with each other).
6. The second filial (F2) generation consistently showed a mix of tall and short plants, in a remarkably precise ratio of approximately 3 tall : 1 short. The "short" trait reappeared.

Mendel's Deductions and Core Concepts:
- Discrete Units of Heredity (Genes): Mendel proposed that hereditary traits are determined by distinct, particulate units, not by blending fluids. We now call these units genes. Each parent contributes one such unit to the offspring.
- Alleles: For each gene, there are different versions, or forms, which Mendel called "factors." We now call these alleles. For instance, the gene for pea plant height has two alleles: one for tallness and one for shortness.
- Diploidy and Allele Pairs: Organisms inherit two alleles for each gene, one from each parent. These two alleles constitute the genotype for that trait.
- Dominance and Recessiveness: Mendel observed that one allele could completely mask the expression of another.
- Segregation: The most crucial part of the law. It states that during the formation of gametes (sex cells: sperm or egg), the two alleles for a heritable character segregate (separate) from each other, so that each gamete receives only one allele.

• Dominance and Recessiveness: Mendel observed that one allele could completely mask the expression of another.
• The allele that expresses its phenotype fully even when paired with a different allele is called the dominant allele (e.g., the allele for tallness, usually represented by a capital letter, 'T').
• The allele whose phenotypic expression is masked in the presence of a dominant allele is called the recessive allele (e.g., the allele for shortness, represented by a lowercase letter, 't'). A recessive trait is only expressed when two copies of the recessive allele are present.

• Segregation: The most crucial part of the law. It states that during the formation of gametes (sex cells: sperm or egg), the two alleles for a heritable character segregate (separate) from each other, so that each gamete receives only one allele. When fertilization occurs, the zygote receives one allele from each parent, re-establishing the pair.