Mendel’s Experiments and the Laws of Inheritance

That Mendel was able to make his discoveries before the discovery of meiosis was due in part to the methods of experimentation he used. Mendel’s work is a fine example of preparation, choice of experimental material, execution and interpretation. Let’s see how he approached each of these steps.

Mendel devised a careful research plan

Mendel chose the garden pea for his studies because of its ease of cultivation, the feasibility of controlled pollination and the availability of varieties with differing traits. He controlled pollination and thus fertilization of his parent plants by manually moving pollen from one plant to another. Thus he knew the parentage of the offspring in his experiments. The pea plants Mendel studied produce male and female sex organs and gametes in the same flower.

If untouched, they naturally self-pollinate—that is, the female organ of each flower receives pollen from the male organs of the same flower. Mendel made use of this natural phenomenon in some of his experiments. Mendel began by examining different varieties of peas in a search for heritable characters and traits suitable for study:

A characteris an observable feature, such as flower color.

A traitis a particular form of a character, such as whiteflowers.

A heritablecharacter trait is one that is passed from parentto offspring.

Mendel looked for characters that had well-defined, contrasting alternative traits, such as purple flowers versus white flowers. Furthermore, these traits had to be true-breeding, meaning that the observed trait was the only form present for many generations. In other words, peas with white flowers, when crossed with one another, would have to give rise only to progeny with white flowers for many generations; tall plants bred to tall plants would have to produce only tall progeny. Mendel isolated each of his true-breeding strains by repeated inbreeding (done by crossing of sibling plants that were seemingly identical or by allowing individuals to selfpollinate) and selection. In most of his work, Mendel concentrated on the seven pairs of contrasting traits. Before performing any experimental cross, he made sure that each potential parent was from a true-breeding strain— an essential point in his analysis of his experimental results. Mendel then collected pollen from one parental strain and placed it onto the stigma (female organ) of flowers of the other strain whose anthers were removed. The plants providing and receiving the pollen were the parental generation, designated P. In due course, seeds formed and were planted. The seeds and the resulting new plants constituted the first filial generation, or F1. Mendel and his assistants examined each F1 plant to see which traits it bore and then recorded the number of F1 plants expressing each trait. In some experiments the F1 plants were allowed to self-pollinate and produce a second filial generation, F2. Again, each F2 plant was characterized and counted. In summary, Mendel devised a well-organized plan of research, pursued it faithfully and carefully, recorded great amounts of quantitative data, and analyzed the numbers he recorded to explain the relative proportions of the different kinds of progeny. His results and the conclusions to which they led are the subject of the next several sections.