The bead theory is a disproved hypothesis that genes are arranged on the chromosome like beads on a necklace. According to this theory, the existence of a gene as a unit of inheritance is recognized through its mutant alleles. A mutant allele affects a single phenotypic character, maps to one chromosome locus, gives a mutant phenotype when paired, and shows a Mendelian ratio when intercrossed.
This theory was first proposed by Thomas Hunt Morgan after discovering genes through his work with breeding red- and white-eyed fruit flies. Guido Pontecorvo continued to work under the basis of this theory until Seymour Benzer showed in the 1950s that the bead theory was not correct. He demonstrated that a gene can be defined as a unit of function. A gene can be subdivided into a linear array of sites that are mutable and that can be recombined. The smallest units of mutation and recombination are now known to be correlated with single nucleotide pairs.
Several tenets of the bead theory are worth emphasizing:
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