Bead theory

The bead theory is a disproved hypothesis that genes are arranged on the chromosome like beads on a necklace. This theory was first proposed by Thomas Hunt Morgan after discovering genes through his work with breeding red and white eyed fruit flies.{{Cite journal|last=Morgan|first=Thomas Hunt|date=1910|title=Sex Limited Inheritance in Drosophilia|url=http://www.esp.org/foundations/genetics/classical/thm-10a.pdf|journal=Science|volume=32|issue=812|pages=120–122|via=ESP.org|doi=10.1126/science.32.812.120|pmid=17759620|bibcode=1910Sci....32..120M}} 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. 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.{{Cite journal|last=Siddoqi|first=Obaid|date=1999|title=Guido Pontecorvo (1907-1999)|journal=Journal of Genetics|volume=78|issue=3|pages=171–173|doi=10.1007/BF02934463|s2cid=31960838}} 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.