The Millikan oil-drop experiment (1909), performed by Robert A. Millikan and Harvey Fletcher to measure the charge of the electron. In the first step, the terminal velocity of an oil drop was measured, which means that the drag force (which can be calculated using Stokes' law) equals the force of gravity. From this, the mass can be calculated. Then, by turning on an electric field, the particle starts to move upward with a terminal velocity when the electric force balances out the forces of gravity and drag. Using this, and the mass of the drop, the total charge on the drop can be calculated. Millikan and Fletcher found that the total charge on the drops were always quantized—that is, always an integer multiple of some constant; specifically, the constant they found is about 1.59 × 10-19 coulombs, within 1% of the currently accepted value. Gregor Mendel's experiments with pea plants (1860s) pioneered the studio of genetics. Mendel, an Austrian monk, proposed the law of segregation, which holds that each organism has two alleles for each trait, which are segregated into gametes so that each gamete inherits one copy, as well as the law of independent assortment, which says that genes for individual traits are inherited independently. He supported both of those laws with experimental evidence by growing and counting pea plants. He worked with seven characteristics including plant height, seed shape, and color. His experiments primarily consisted of hybridizing plants with certain characteristics, and observing what fraction of the next generation had certain traits. His results were remarkably close to the values that would be predicted from modern genetics—close enough, in fact, that Mendel has been accused of manipulating his data.