Gregor Johann Mendel (1822-1884), an Austrian Augustinian monk and botanist, discovered the laws of heredity, and the basis of genetics, from his experiments with garden peas. His work, which was unknown to Darwin, supplied a mechanism for the inheritance of random variations. It was ignored during his lifetime and was not rediscovered until 1900.

The Swedish stamps above offer insights into the Nobel prize winning work done by the scientists listed below:

Thomas Hunt Morgan (1866-1945) was aware of Mendel's work, which had just been rediscovered, and performed his own genetic experiments, not with peas, but with fruit flies, which could reproduce rapidly and in great numbers and had only four pairs of chromosomes. He found that Mendel's Law of the independent assortment of characters was true, but that in some cases a linkage existed between characters. The extent of linkage between characters was a measure of their position, or nearness to each other, on the same chromosome. Mapping the genes and their positions in the chromosomes explained the range of Mendelian results. Morgan won the 1933 Nobel prize in physiology or medicine.

Francis Crick, James D. Watson, and Maurice Wilkins determined the molecular structure of DNA as a double helix or two intertwined spirals of phosphate and sugar molecules linked by pairs of organic bases. The sequences of the paired organic bases form the genetic code of an organism. The double helix structure is an eye-catching symbol that appears on many stamps, including the Czech Mendel stamp above, and examples from Liechtenstein and Israel below.

Barbara McClintock's lifelong research interest was in the genetics of maize. She studied individual chromosomes and related them to the physical characteristics of the kernels on the cob, finding that changes in the color pattern of the kernels corresponded to the transposition of structural elements of the chromosomes, or "jumping genes." She made this discovery before the structure of DNA was known.