Overview
Gregor Mendel, a friar in Brünn, Austria (now Brno, Czech Republic), crossed pea plants in his monastery garden and presented the results to the Brünn Natural History Society in 1865; they were printed in 1866 as "Experiments on Plant Hybridization". The work drew little attention until 1900, when Hugo de Vries, Carl Correns and Erich von Tschermak independently reached the same results. William Bateson then publicized it in Mendel's Principles of Heredity (1902).
Experimental design
Peas self-pollinate, so true-breeding lines are easy to obtain; crosses can be controlled by hand-pollination; and they show clear-cut traits such as round or wrinkled and yellow or green seeds. Mendel chose seven pairs of contrasting traits, counted offspring across several generations and analysed the ratios, a quantitative approach that was new at the time.
Law of dominance
Crossing true-breeding round-seeded and wrinkled-seeded plants gave a first hybrid generation (F1) with only round seeds. The trait that appears in the F1 is called dominant and the one that disappears recessive. Korean textbooks teach this as the "law of dominance", but because dominance is not always complete, many modern texts count only segregation and independent assortment as laws.
Law of segregation
When the F1 (Rr) self-pollinates, the second generation (F2) shows round and wrinkled seeds in roughly a 3:1 ratio. Mendel explained this by proposing that each plant carries two hereditary factors (today's alleles) for each trait, and that the two separate when gametes form, each gamete receiving one. The F2 genotypes are therefore RR:Rr:rr = 1:2:1. A test cross with a recessive homozygote (rr) reveals the genotype of a dominant-looking plant: a heterozygote (Rr) gives offspring in a 1:1 ratio.
Law of independent assortment
In dihybrid crosses following two traits at once, such as seed shape and colour (RrYy), the F2 phenotypes came out at 9:3:3:1. Each trait still segregates 3:1, and the combined ratio is the product of the two, which means genes for different traits enter gametes independently of each other.
Exceptions and significance
- Incomplete dominance: in four-o'clock flowers, a red × white cross gives pink F1 plants.
- Codominance: both alleles are expressed, as in blood type AB in the ABO system.
- Linkage: genes that sit close together on the same chromosome tend to be inherited together, so independent assortment fails; Morgan's fruit-fly work (1911) showed this.
- Polygenic inheritance: traits such as height, shaped by many genes, vary continuously.
Even so, Mendel's laws showed that heredity is passed on in particle-like units rather than as blending fluids, and they became the starting point of modern genetics. The question of how far traits are set by genes alone leads to the nature versus nurture debate, and the long neglect and later rediscovery of Mendel's work illustrates the tentativeness of science.
Sources reviewed: 2026-10-16
References
- Mendel, G. (1866). Versuche über Pflanzen-Hybriden. Verhandlungen des naturforschenden Vereines in Brünn, 4, 3–47.
- Bateson, W. (1902). Mendel's Principles of Heredity: A Defence. Cambridge: Cambridge University Press.
- Morgan, T. H. (1911). Random segregation versus coupling in Mendelian inheritance. Science, 34(873), 384.