Discrete Genes Are Inherited: Gregor Mendel (1 of 2)

Throughout the nineteenth century, heredity remained a puzzle to scientists. How was it that children ended up looking similar to, but not exactly like, their parents? These questions fascinated and frustrated Charles Darwin deeply. After all, heredity lies at the heart of evolution. The variations in each generation are the raw material for natural selection, while the continuity from one generation to the next allows the changes wrought by natural selection to have long-term effects. Darwin himself proposed that each cell in an animal’s body released tiny particles that streamed to the sexual organs, where they combined into eggs or sperm. They would then blend together when the animal mated. But “pangenesis,” as Darwin called it, didn’t hold up to scrutiny.

Ironically, it was just as Darwin was publishing the Origin of Species that someone got the first real glimpse of the biological machinery behind heredity. In a secluded monastery in what is now the Czech Republic, a monk named Gregor Mendel was studying heredity in a garden of peas. Mendel, the son of a farmer, had always been interested in plants, and while at the University of Vienna he had been trained in mathematics and learned how to design experiments and analyze data. In the 1850s, he decided to run an experiment to better understand what kept species distinct and what made it possible for hybrids to form. He bred thousands of pea plants and recorded how traits were passed on from one generation to the next.

Trait Inheritance
Mendel selected 22 different varieties of peas and interbred them, keeping track of seven different traits, such as pea texture—smooth or wrinkled. Mendel found that when he hybridized smooth and wrinkled peas, he produced peas that were all smooth. But if he then produced a new generation of peas from the hybrids, a quarter of the peas were wrinkled.

Mendel proposed that the peas were not blending their “wrinkled” and “smooth” traits together. Each hybrid pea inherited both traits, but only the smooth trait became visible. In the next generation, the traits were handed down again, and a quarter of the new peas inherited two “smooth” traits, which made them smooth. Mendel had discovered what later scientists called “dominant” and “recessive” alleles.