Bead theory
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. Several tenets of the bead theory are worth emphasizing :- 1. The gene is viewed as a fundamental unit of structure, indivisible by crossing over. Crossing over take place between genes ( the beads in this model ) but never within them. 2. The gene is viewed as the fundamental unit of change or mutation. It changes in toto from one allelic form into another; there are no smaller components within it that can change. 3. The gene is viewed as the fundamental unit of function ( although the precise function of gene is not specified in this model ). Parts of a gene, if they exist cannot function. 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.
References
- BENZER S (1956). "Genetic fine structure and its relation to the DNA molecule". Brookhaven Symp Biol (8): 3–5. PMID 13293416.
- An Introduction to Genetic Analysis 7th edition Griffiths AJF, Miller JH, Suzuki DT, et al.
New york W.H. Freeman;2000