There are plants that are tall and can transmit only Tallness to offspring. There are plants that are dwarf and can transmit only Dwarfness to offspring. So too, there are plants of white blossom or of coloured blossom that can transmit, respectively, only White or Coloured blossoming to offspring.
When a Tall is crossed with a Dwarf plant, however, or a Coloured with a White plant, strange to say, the hybrid offspring of this cross shows one only of these opposite traits, to the exclusion of the other. No intermediate, or mixed, forms are produced.
Thus, a Tall crossed with a Dwarf produces only Talls. Plants of Coloured flower crossed with those of White flower give only Coloured flowering varieties. A yellow and a green-seeded cross produce only yellow-seeded plants.
In the cross between plants of opposite traits, one set of traits appears thus, exclusively, in the hybrid offspring. These traits—because they dominate growth and development—Mendel styled "Dominant." While those traits which are dominated by the other and opposite traits and do not appear in offspring, he styled "Recessive."
On further breeding, a new and stranger thing happens, however. Because when such hybrids—plants bred of parents that had borne, respectively, "Dominant" and "Recessive" characteristics, but with the parental Dominant traits so overpowering the Recessive traits of the other parent that these latter are submerged and concealed—When these hybrids are crossed with other hybrids like themselves, both the Dominant and the Recessive traits of the original parents reappear in offspring. The tall hybrids resulting from the cross between Tall and Dwarf plants, when crossed with other tall hybrids of similar origin, produce both Tall and Dwarf plants. So with Colour, and with the other so-called "Contrasted Traits."
It becomes evident, therefore, that although the Dominant traits of Tallness and Colour overpower in the growth and development of the second generation of plants, the Recessive traits of Dwarfness and Whiteness, these latter traits are submerged only, and are neither impaired in their values, nor destroyed. In the third generation, under different conditions of mating, the original Recessive, and submerged, traits re-appear, and reveal themselves in offspring-plants as the Dwarfness or the Whiteness that had characterised their grandparents.
Mendel assumed that such hybrid plants—offspring of a Dominant and of a Recessive parent—produce two varieties of sex-cells, or gametes, and that one order of cells contain the Dominant traits of the Dominant parent, while the other order contain the Recessive traits of the Recessive parent.
But any individual sex-cell, or gamete, cannot (according to his view) bear both Dominant and Recessive traits. The Dominant traits and the Recessive traits of the respective parents he regarded as being segregated, absolutely, in one or in the other set of sex-cells produced by hybrid varieties. And of these, the cells bearing Dominant traits are able to transmit Dominant traits only to offspring; while the cells bearing Recessive traits transmit Recessive traits only to offspring.
II
Now, Biology shows that plants and living creatures develop from a single microscopic cell, formed by the union of two half-cells, of which each half was contributed by one of the two parents.