The bead theory is a hypothesis that the gene is arranged on a chromosome like a bead on a necklace. This theory was first proposed by Thomas Hunt Morgan after discovering the gene through his work by breeding red and white eyed fruit flies. According to this theory, the existence of genes as a unit of inheritance is recognized through its mutant allele. A mutant allele affects a single phenotype character, maps to a chromosome locus, provides a mutant phenotype when paired and shows a Mendelian ratio when crossed. Some of the principles of manic theory should be emphasized: - 1. These genes are seen as basic units of structure , can not be subdivided by crossing. Crossing takes place between genes (beads in this model) but never in it. 2. This gene is seen as a unit of change or fundamental mutation . Toto changes from one form of allele to another; there are no smaller components in it that can change. 3. This gene is seen as basic functional unit (although proper gene function is not specified in this model). Part of the gene, if they exist can not work. Guido Pontecorvo continued to work under the basis of this theory until Seymour Benzer pointed out in the 1950s that the bead theory was incorrect. He points out that genes can be defined as unit functions. A gene can be divided into linear arrays of changeable and merged sites. The smallest mutation and recombination units are now known to correlate with single nucleotide pairs.
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References
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BENZER S (1956). "Genetically fine structure and its relationship to DNA molecules". Brookhaven Symp Biol (8): 3-5. PMID 13293416. - Introduction Genesis Analysis 7th Edition Griffiths AJF, Miller JH, Suzuki DT, et al.
New york W.H. Freeman, 2000
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