Gene
as the functional unit of inheritance
A gene is a basic
physical and functional unit of heredity. The concept of the gene was first
explained by Gregor Mendel in 1860’s. He never used the term
‘gene’. He called it ‘factor’. In 1909, the Danish biologist Wilhelm
Johannsen, coined the term ‘gene’, that was referred to discrete determiners of
inherited characteristics.
According to the
classical concept of gene introduced by Sutton in 1902, genes have been defined
as discrete particles that follow Mendelian rules of inheritance, occupy a
definite locus in the chromosome and are responsible for the expression of
specific phenotypic character. They show the following properties:
·
Number of genes in each organism is more than the number of
chromosomes; hence several genes are located on the same chromosome.
·
The genes are arranged in a single linear order like beads on a
string.
·
Each gene occupies a specific position called locus.
·
Genes may exist in several alternate forms called alleles.
·
Genes may undergo sudden change in positions and composition
called mutations.
·
Genes are capable of self-duplication producing their own copies.
The experiments of George Beadle and
Edward Tatum in the early 1940’s on Neurospora crassa (the red bread mould) led
them to propose one gene-one enzyme hypothesis, which states that one gene
controls the production of one enzyme.
It was observed that an enzyme may
be composed of more than one polypeptide chain and a gene can code for only one
polypeptide chain. Thus one gene-one polypeptide hypothesis states that one
gene controls the production of only one polypeptide chain of an enzyme
molecule.
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