Chromosomal Theory of Inheritance

Charles Darwin first conceived the idea of hereditary units when he published his theory of pangenesis in 1868. In this model, circulating units called gemmules are accumulated in the gonads and transmitted to the off-spring. This theory was discredited by experimental tests performed by Francis Galton in the 1870s. Galton used blood transfusions in rabbits to show that the alleged gemmules in one rabbit's blood did not alter the heredity of the recipient rabbit's blood. In the 1890s Hugo de Vries took the term "pangenesis" and trimmed it to "pangene" for the assumed units of inheritance. He argued that pangenes remained inside the cell and did not migrate. It was this theory of intracellular pangenesis that led de Vries to independently find what Gregor Mendel had discovered thirty years earlier in his work with contrasting traits in garden peas—there are units of inheritance that are transmitted by reproduction. Wilhelm Johansson introduced the term "gene" to replace several contending and misleading terms for the basic unit of heredity in 1909. The term "genetics" came earlier, when William Bateson coined the word in 1906 to represent the new field that studied heredity, variation, and evolution. The terms "gene," "genetics," and the biblical term "genesis" all share a common Latin root, gen, meaning origin.

Mendel identified what he called "factors" (later called genes) as the underlying cause for the appearance of certain traits in peas. He described them as stable units that seemed to disappear in a hybrid (a plant grown from a cross between two parent plants that show differing traits) but would reappear among some of the progeny of such hybrids. Mendel identified two laws—the law of segregation and the law of independent assortment—which together governed the movement of factors from parent to progeny. This strongly suggested that the factors of inheritance were discrete physical objects. Shortly before Mendel's work was being rediscovered, advancements in the construction of microscopes had allowed scientists to make careful observations of cell division. This led to the discovery of colored bodies in the cell nucleus that appeared to double and divide just before each division. These were called chromosomes ("colored bodies").

By 1902 the chromosome movements during meiosis had been worked out, and Walter Sutton used them to explain Mendel's laws. He argued that the pairing and separation of homologues would lead to the segregation of a pair of factors they carried. Thus, to use one of Mendel's own experiments, hybridizing yellow and green pea plants would yield one yellow and one green gamete apiece. (A gamete is germ cell, sperm or egg, that contains half of a full complement of chromosomes, originating from one of the parent plants.) The result is a gametic ratio of 1:1. The union of pollen and ovules would result in the 3 yellow to 1 green Mendelian ratio. Similarly, two different pairs of homologs would yield the 9:3:3:1 ratio associated with Mendel's law of independent assortment. Sutton called his union of cytology with Mendelian breeding analysis the "chromosome theory of heredity."