The father of modern genetics is Gregor Mendel, a monk. In the late 19th century, Mendel performed a series of experiments on pea plants. By crossing pea plants with different characteristics, Mendel discovered the characteristics of genes from observing the plants' offspring.
Mendel found that certain characteristics were dominant over others. These characteristics are governed by units later found to be genes. The differences in characteristics come from different forms of genes called alleles.
Pea plants receive one allele from each parent for each characteristic. If the plant has at least one dominant allele, it displays the dominant characteristic. If the plant contains two of the non-dominant allele, called the recessive allele, then it displays the recessive characteristic.