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Chromosomal Theory of Inheritance

Further Advances In Theory

Also from 1913 to 1916, Bridges found some exceptions to the expected modified 3:1 ratio for white-eyed flies. He inferred, and confirmed by microscopic examination of cells, that these unexpected departures arose from the failure of homologous chromosomes to separate during meiosis. Bridges called this phenomenon nondisjunction and used it as a proof of the chromosome theory of heredity.

While both Mendel and Morgan's group worked with simple, single-gene traits, the relation of genes to most character traits turned out to be more complex. A successful analysis of this was presented by Muller. He argued that the variable wing shapes and lengths of beaded and truncated wings in fruit flies involved several factors. A chief gene was essential, but it required modifier genes that could intensify or diminish its expression. By combining different modifier genes and the chief gene in two parents, Muller could predict the percentages of wing shape and length among the progeny. Muller used this analysis to support a Darwinian model of natural selection of character traits whose variations owe their origins to the highly heterozygous state of natural populations and to new mutations that arise in each generation. Muller's analysis added evolution to cytology and breeding analysis as the three tributaries of classical genetics.

Elof Carlson


Allen, G. E. Thomas Hunt Morgan: The Man and His Science. Princeton, NJ: Princeton University Press, 1978.

Judson, H. P. The Eighth Day of Creation: The Makers of the Revolution in Biology. New York: Simon & Schuster, 1979.

Morgan, Thomas Hunt, et al. The Mechanism of Mendelian Heredity. New York: Holt Reinhart & Winston, 1915. Reprinted by Johnson Reprint Corporation with an Introduction by Garland E. Allen, 1978.

Sturtevant, Alfred Henry. A History of Genetics. New York: Harper & Row, 1965.

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Medicine EncyclopediaGenetics in Medicine - Part 1Chromosomal Theory of Inheritance - The Birth Of A Science, X-linked Inheritance In Hybrids, Further Advances In Theory