Gene Definition

In 1909, Johanssen introduced the term gene. Before him, Mendel had given the term ‘factor’ for the unit of inheritance that is responsible for expressing a trait. Johanssen defined the gene as ‘the elementary unit of inheritance which can be assigned to a particular trait.’ The work of Morgan proposes that a gene is the shortest segment of a chromosome that can be separated by crossing over.

Gene is presently defined as ‘the unit of inheritance composed of a segment of DNA situated at a specific locus that carries coded information and also has the ability to undergo mutation and crossing over.’

The definition of a gene states that it is the basic physical and functional unit of heredity. Genes, structurally are made up of nucleotides called DNA.

Three nucleotides together codes for amino acid and a string of amino acids in a particular sequence forms a functional protein. These proteins may function as enzymes and control the metabolism. Archibald Garrod studied the errors of metabolism and found that genes operate through enzymes. Beadle and Tatum in 1948 proposed the one-gene one-enzyme hypothesis. But some enzymes require more than one polypeptide. Therefore it is modified into the one-gene one-polypeptide hypothesis.

Every diploid organism has two copies of genes derived from each of its parents. They are expressed in a dominant or recessive manner.

The complete set of all genes present in a cell is known as a genome. There are 30000 genes in human genome. The largest gene in the human genome is 2400 kbp long and the smallest gene in the human genome is 14 bp long.

Main Article: Genes

Further Reading:

• Genetics
• Chromosome

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