Chromosomal Theory of Inheritance

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.