Gene Concept,quot,Forms,Beads- On-a-String Concept,Genetic Materials,Wilhelm Johansen,quot Gene quot

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	Gene Concept - Gene is the basic functional unit of life. Just as atoms are the functional units in chemistry, so are genes in biology. The atom combines in n number of ways to form the massive, complex structures like mineral ore, etc. Similarly the genes combine in n forms to form complex individuals. Just like the atoms combine and exchange electrons from one group to the other to form complex atoms, genes also combine with one another to form complex genes, thus providing a scope for complex gene structure formation.
The concept of gene was first introduced by Mendel and he termed them as hereditary factors or elements. This concept was purely hypothetical and did not carry any experimental evidence. Wilhelm Johansen coined the term gene in 1909 to describe heritable factors responsible for the transmission and expression of a given biological character, but without reference to any particular theory of inheritance. But evidence generated from experimental work carried out with higher plants suggest genes as the units of inheritance, controlling one phenotypic trait. Based on this data, Morgan in 1926 proposed the particulate gene theory in which he stated that genes are corpuscular and arranged in a linear order on chromosomes like beads on a string.
According to the pre 1940 beads-on-a-string concept, the gene was the basic unit of inheritance defined by three criteria function, recombination and mutation pair. Since it clearly does not make sense to call each nucleotide pair a gene, emphasis has been shifted to the original definition of the gene as the unit of function. Thus, Mendel's unit of inheritance co relates more directly with the gene as a unit of function than with the unit of structure or single nucleotide pair. 1. The unit of function means that a fragment or unit of genetic material controls the inheritance of one unit character or attribute of phenotype. 2. The unit of structure could be operationally defined in two ways
i As the unit of inheritance not sub divisible by recombination. ii. The smallest unit of genetic material capable of independent mutation. The classical view was that all three criteria defined the same basic unit of inheritance. But this theory was discarded with advances in the DNA structure. Then Sutton introduced a new gene concept, which was elaborated by Muller. This concept was known as classical gene structure. It states that 1. Genes determine physical as well as physiological characteristics.
	2. Genes are present on the chromosome, and on a single chromosome there are many genes. 3. Genes occupy a specific position on a chromosome which is called as locus or loci and genes are arranged in single linear order. .