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Genetics and Genomics Timeline
Theodor Boveri (1862-1915) and Walter Sutton (1877-1916) propose that chromosomes bear hereditary factors in accordance with Mendelian laws

Rediscovery of Mendel's laws in 1900 clarified inheritance. But Mendel had worked with traits of whole organisms. He did not investigate how characteristics are sorted and combined on a cellular level, where reproduction takes place. In 1902, the German scientist Theodor Boveri and the American Walter Sutton, working independently, suggested that chromosomes could be shown to bear the material of heredity. Mendelian concepts, as it turned out, had an excellent fit with facts about chromosomes.

Walter Sutton
Boveri had previously shown that chromosomes remain organized units through the process of cell division, and he demonstrated that sperm and egg cells each contribute the same number of chromosomes. But does each chromosome have specific properties? Is a full complement of chromosomes necessary for reproduction and development?

In a series of experimental manipulations with sea urchin eggs, Boveri demonstrated that individual chromosomes uniquely impact development. Sea urchin eggs can be fertilized with two sperm. Boveri showed that daughter cells of such double unions possess variable numbers of chromosomes. Of the embryos that result, Boveri found that only the small percentage—about 11 percent—possessing the full set of 36 chromosomes would develop normally. A "specific assortment of chromosomes is responsible for normal development," wrote Boveri in 1902, "and this can mean only that the individual chromosomes possess different qualities."

In addition, Boveri recognized the Mendelian concepts of segregation and assortment could be interpreted to operate on a cellular level, with chromosomes containing the "factors"—as Mendel called the genes. The probability was "extraordinarily high," wrote Boveri in 1903, "that the characters dealt with in Mendelian experiments are truly connected to specific chromosomes."

Theodor Boveri

An American graduate student, Walter Sutton, came to the same conclusion at about the same time. Sutton, working with marine life forms, had also become familiar with the process of "reduction division" (later called meiosis), which gives rise to reproductive germ cells, or gametes. In meiosis, the number of chromosomes is reduced by half in sperm and egg cells, with the original number restored in the zygote, or fertilized egg, during reproduction. This process was consonant with Mendel's idea of segregation. In 1902 Sutton suggested that "the association of paternal and maternal chromosomes in pairs and their subsequent separation during the reduction division...may constitute the physical basis of the Mendelian law of heredity." His "The Chromosomes in Heredity" was published in 1903.

The Boveri-Sutton Chromosome Theory, as it came to be known, was discussed and debated during the first years of the twentieth century. It was embraced by some but strongly rejected by others. By 1915 Thomas Hunt Morgan—initially a strong skeptic—laid the controversy to rest with studies of the fruit fly Drosophila melanogaster.

Baltzer, F. Theodor Boveri. Science 144, 809-815 (1964).
McKusick, V.A. Walter S. Sutton and the physical basis of Mendelism. Bull Hist Med 34, 487-497 (1960).

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