Gregor Johann Mendel (1822-1884) was an Augustinian friar and scientist in Brno (now Czech Republic) whose meticulous experimental work with garden peas between 1856 and 1863 established the foundational principles of heredity that were later recognised as the foundation of modern genetics. Mendel conducted approximately 29,000 pea plants across eight years of experiments, using his mathematical background to quantitatively analyse the inheritance patterns he observed and derive statistical ratios — an approach unprecedented in biology at the time. He published his results in 1866 in "Versuche über Pflanzenhybriden" (Experiments on Plant Hybridisation), a paper that was largely ignored by the scientific community until 1900, when three scientists (de Vries, Correns, and von Tschermak) independently rediscovered similar results and recognised Mendel's prior work, triggering the "rediscovery of Mendel" and the establishment of genetics as a scientific discipline.

From his pea experiments, Mendel formulated three fundamental laws: Law of Dominance: when two contrasting alleles are present in an organism (heterozygous), only one (dominant) is expressed in the phenotype; the other (recessive) is masked but not lost, and can reappear in later generations. Law of Segregation (Law of Purity of Gametes): the two alleles of each gene segregate (separate) during gamete formation, so each gamete contains only one allele for each trait. These alleles reunite randomly during fertilisation. Demonstrated by the 3:1 F2 phenotypic ratio in monohybrid crosses. Law of Independent Assortment: alleles of different genes (on different chromosomes or far apart on the same chromosome) assort independently during gamete formation. Demonstrated by 9:3:3:1 F2 phenotypic ratio in dihybrid crosses. Note: Mendel's third law has exceptions when genes are linked (on the same chromosome), as discovered by Morgan and Sturtevant.

The monohybrid cross — crossing plants differing in a single trait — was Mendel's fundamental experimental approach. Using stem height as the example: P generation (parental): pure-breeding tall (TT) × pure-breeding dwarf (tt). TT plants produce only T gametes; tt plants produce only t gametes. F1 generation: all offspring receive T from the tall parent and t from the dwarf parent → all Tt (heterozygous). Phenotype: all tall (because T is dominant over t). This result demonstrates the Law of Dominance — in Tt plants, T is expressed and t is suppressed. F1 self-cross (F1 × F1, Tt × Tt): each Tt plant produces both T and t gametes in equal proportions (1/2 T gametes, 1/2 t gametes). Random fertilisation: TT (1/4), Tt (2/4), tt (1/4). Genotype ratio: 1 TT : 2 Tt : 1 tt. Phenotype ratio: 3 tall (TT + Tt) : 1 dwarf (tt) = 3:1. The reappearance of the dwarf phenotype in F2 (in a predictable 1/4 proportion) confirmed that the recessive character was NOT destroyed in F1 but only masked — a profound insight supporting the particulate theory of inheritance against the then-prevailing blending inheritance hypothesis.

While Mendel described dominance as a phenotypic pattern (dominant trait is expressed when at least one dominant allele is present), the molecular mechanisms underlying dominance vary by gene and trait. Haploinsufficiency: the recessive phenotype appears when only one functional copy of the gene is present because one copy produces insufficient protein for normal phenotype — NOT the typical Mendelian pattern where one copy IS sufficient (this is why most loss-of-function mutations are recessive — one functional copy produces enough protein). Complete dominance (classical Mendelian): one functional allele produces sufficient protein/enzyme product for full phenotype expression, so the heterozygote looks identical to the dominant homozygote. For stem height in peas, the dominant T allele encodes a functional enzyme (Le enzyme, a gibberellin biosynthesis enzyme) producing sufficient GA to give full stem elongation; the recessive t allele has a loss-of-function mutation reducing GA, and one functional T allele in Tt plants produces enough GA for full height. Incomplete dominance (non-Mendelian): neither allele is fully dominant — F1 hybrid shows an intermediate phenotype (e.g., red × white → pink in snapdragons, 1:2:1 ratio in F2 rather than 3:1). Codominance: both alleles are fully expressed simultaneously in the heterozygote (e.g., AB blood type where both IA and IB alleles are fully expressed).