Gregor Mendel (1822-1884) was a monk of the Augustinian monastery of St. Thomas in Brno, Moravia (now the Czech Republic). In the mid-1850s, Mendel became interested in hybridization and began breeding garden peas. He analyzed the hybrid offspring of parents that showed contrasting forms of a single trait - for example, green seeds versus yellow seeds. Mendel reasoned that each trait is determined by a pair of "factors" with each parent contributing one factor.
In an 1865 publication, Mendel showed that factors are discrete bits of hereditary information that remain unchanged through generations. The dominant trait - yellow seeds in this case - appears whenever one of the inherited factors is dominant. The recessive trait, green seeds, appears only when both inherited factors are recessive.
During the first decade of the 20th century, Mendel's work was rediscovered, his "factors" were renamed "genes," and his ideas were rapidly absorbed into animal and plant breeding. The British physician Archibald Garrod showed that Mendel's laws applied equally well to human metabolic traits. Examples of dominant (brachydactyly and congenital cataracts) and recessive disorders (alkaptonuria and albinism) were identified in humans.
To help the lay public understand the Mendelian basis of human heredity, eugenicists frequently used eye color as an example. In 1907, Charles and Gertrude Davenport published their finding that human eye color was inherited just like pea color; genes for dark eyes were dominant over genes for light eyes. It is still the most frequently used example of human heredity, even though scientists now believe eye color determination is more complex. Instead of a single pair of genes determining eye color, two or three sets of genes are thought to be involved.
The unexpected complexity of something as trivial as eye color gives one an idea how wrongheaded eugenicists were when they used simple Mendelian laws to explain human behaviors such as pauperism, mental retardation, psychosis, vagrancy, alcoholism and criminality.