In the mid-$19^\text{th}$ century, the mechanism of heredity was poorly understood, often assumed to be a process of blending parental characteristics. The Austrian monk Gregor Mendel (1822–1884), through meticulous, quantitative breeding experiments conducted on thousands of pea plants (Pisum sativum) in his monastery garden, discovered the fundamental principles that govern how traits are passed from one generation to the next. His work, though ignored in his lifetime, was rediscovered in 1900 and became the bedrock of modern Genetics.

Mendel’s success was due to his experimental methodology:

• Choice of Subject: Pea plants were ideal because they could be easily cross-pollinated and had short generation times.

• Discrete Traits: He focused on traits that appeared in two distinct, non-blending forms (e.g., tall vs. short, purple flowers vs. white flowers).

• Purebred Lines: He began his experiments using only purebred lines that consistently produced the same trait over many generations.

Mendel's initial experiments tracked a single trait at a time (monohybrid crosses). He crossed two contrasting purebred parent plants (the $P$ generation), such as a purebred tall plant with a purebred short plant.

• $\mathbf{F_1}$ Generation: All offspring in the first filial ($F_1$) generation were uniform, exhibiting only one parental trait (e.g., all were tall). The other trait (shortness) vanished.

• $\mathbf{F_2}$ Generation: When the $F_1$ generation was self-pollinated, the vanished trait reappeared in the second generation ($F_2$) in a precise and predictable $3:1$ ratio (e.g., 3 tall plants for every 1 short plant).

Mendel concluded that traits are governed by discrete, particulate units (which he called "factors," now known as alleles or genes) that do not blend and are separated during reproduction.

Law of Segregation: The two alleles for a heritable character separate (segregate) during gamete formation and end up in different gametes. Each gamete carries only one allele for each trait.

Mendel then tracked two different traits simultaneously (dihybrid crosses, e.g., seed shape and seed color).

• The Finding: He found that the inheritance of one trait (like seed color) was completely independent of the inheritance of the other trait (like seed shape). The alleles for the two traits were shuffled randomly.

• The Ratio: This independent shuffling resulted in a fixed ratio of four possible phenotypes in the $F_2$ generation: $9:3:3:1$.

Law of Independent Assortment: The alleles of two (or more) different genes sort independently of one another into gametes. (This applies to genes located on different chromosomes or genes far apart on the same chromosome.)

Mendel's work was overlooked for 35 years until its simultaneous rediscovery in 1900. His findings provided the mathematical, probabilistic basis for heredity, replacing speculative theories with a precise, quantitative science. His discovery proved that inheritance is governed by discrete units—the gene—which became the central concept of all $20^\text{th}$-century biology, from the study of chromosomes to the discovery of $\text{DNA}$ (Article 172).

In Conclusion: Gregor Mendel is the founder of genetics. Through his meticulous pea plant experiments, he established the fundamental Laws of Segregation and Independent Assortment. By proving that heredity is mediated by discrete, non-blending units (alleles) whose transmission follows predictable statistical laws, he provided the essential scientific framework for understanding biological variation and the mechanism of evolution.

In the mid-$19^\text{th}$ century, the mechanism of heredity was poorly understood, often assumed to be a process of blending parental characteristics. The Austrian monk Gregor Mendel (1822–1884), through meticulous, quantitative breeding experiments conducted on thousands of pea plants (Pisum sativum) in his monastery garden, discovered the fundamental principles that govern how traits are passed from one generation to the next. His work, though ignored in his lifetime, was rediscovered in 1900 and became the bedrock of modern Genetics.

Mendel’s success was due to his experimental methodology:

• Choice of Subject: Pea plants were ideal because they could be easily cross-pollinated and had short generation times.

• Discrete Traits: He focused on traits that appeared in two distinct, non-blending forms (e.g., tall vs. short, purple flowers vs. white flowers).

• Purebred Lines: He began his experiments using only purebred lines that consistently produced the same trait over many generations.

Mendel's initial experiments tracked a single trait at a time (monohybrid crosses). He crossed two contrasting purebred parent plants (the $P$ generation), such as a purebred tall plant with a purebred short plant.

• $\mathbf{F_1}$ Generation: All offspring in the first filial ($F_1$) generation were uniform, exhibiting only one parental trait (e.g., all were tall). The other trait (shortness) vanished.

• $\mathbf{F_2}$ Generation: When the $F_1$ generation was self-pollinated, the vanished trait reappeared in the second generation ($F_2$) in a precise and predictable $3:1$ ratio (e.g., 3 tall plants for every 1 short plant).

Mendel concluded that traits are governed by discrete, particulate units (which he called "factors," now known as alleles or genes) that do not blend and are separated during reproduction.

Law of Segregation: The two alleles for a heritable character separate (segregate) during gamete formation and end up in different gametes. Each gamete carries only one allele for each trait.

Mendel then tracked two different traits simultaneously (dihybrid crosses, e.g., seed shape and seed color).

• The Finding: He found that the inheritance of one trait (like seed color) was completely independent of the inheritance of the other trait (like seed shape). The alleles for the two traits were shuffled randomly.

• The Ratio: This independent shuffling resulted in a fixed ratio of four possible phenotypes in the $F_2$ generation: $9:3:3:1$.

Law of Independent Assortment: The alleles of two (or more) different genes sort independently of one another into gametes. (This applies to genes located on different chromosomes or genes far apart on the same chromosome.)

Mendel's work was overlooked for 35 years until its simultaneous rediscovery in 1900. His findings provided the mathematical, probabilistic basis for heredity, replacing speculative theories with a precise, quantitative science. His discovery proved that inheritance is governed by discrete units—the gene—which became the central concept of all $20^\text{th}$-century biology, from the study of chromosomes to the discovery of $\text{DNA}$ (Article 172).

In Conclusion: Gregor Mendel is the founder of genetics. Through his meticulous pea plant experiments, he established the fundamental Laws of Segregation and Independent Assortment. By proving that heredity is mediated by discrete, non-blending units (alleles) whose transmission follows predictable statistical laws, he provided the essential scientific framework for understanding biological variation and the mechanism of evolution.