E-Mail IMAGE: Fluorescence image of two zebrafish embryos - After switching off the Plexin-A1 gene (bottom), malformations of the nervous system occur, such as a pathological enlargement of the neural fluid spaces... view more Credit: Dr. Gabriel Dworschak / University of Bonn When the Augustinian monk Gregor Mendel crossed white-flowering with purple-flowering pea plants in the mid-19th century, he made an interesting discovery: The offspring were all purple. He therefore called this trait dominant, while the white blossom color was recessive. The reason for this phenomenon: In peas, each gene occurs twice. One version comes from the maternal plant and the other from the paternal plant. If a pea has inherited the gene for purple flower color from one parent, but the gene for white flower color from the other, purple wins. Only when two genes for white flowers come together in the offspring plant is it white.