Chromosomes, Mutation, and the Birth of Modern Genetics:
Thomas Hunt Morgan (1 of 2)

In 1900 several scientists across Europe came to the same realization about heredity that Mendel had some 40 years before. But they arrived at the discovery from a very different direction.

Chromosomes, the dark structures in this image, are copied and distributed to the daughter cells as this plant cell reproduces.

Chromosomes Contain Genetic Material
Nineteenth century cell biologists discovered that animal and plant cells had a central compartment known as the nucleus. Each nucleus contained a set of rod-shaped structures, and when a typical cell divided, a new nucleus complete with a new set of rods was created. These rods were named chromosomes for the way they absorbed colored stains. But sperm and eggs contained only half the normal set of chromosomes. When a sperm fertilized an egg, the chromosomes combined to create a full complement.

Scientists realized that the chromosomes stored the information necessary for building an individual, and heredity consisted of the transfer of that information from generation to generation. Each chromosome contained information for many different traits, and scientists dubbed each chromosomal chunk that was responsible for a particular trait a “gene.“

Rediscovering Mendel
Dutch botanist Hugo DeVries and several other scientists carried out breeding experiments in the late 1890s and rediscovered Mendel’s three-to-one ratio. But this new generation could offer a clearer interpretation of what was happening in their experiments. We each carry two copies of the same gene, one from each parent, but in many cases only one copy produces a trait while the action of the other is masked. Here was the secret behind Mendel’s three-to-one ratio of smooth and wrinkled peas.

• Cell images courtesy of Florida State University Molecular Expressions Microscopy Gallery.