Concept of Genes

Mendel’s hereditary “factors” later came to be known as genes, a term coined by W. Johannsen in 1909.

Earlier, genes were considered as bead-like units arranged linearly on chromosomes.

Initially, a gene was regarded as the following:

• a unit of function called cistron,
• a unit of mutation called muton,
• a unit of recombination called recon.

The concept of a gene changed with advances in molecular genetics.

Important stages in this change:

• One gene-one enzyme hypothesis: Beadle and Tatum proposed that one gene controls the production of one enzyme.
• One gene-one polypeptide hypothesis: Later it was understood that one gene usually codes for one polypeptide chain, not necessarily a whole enzyme.
• Some genes also code for rRNA and tRNA, not only polypeptides.
• In some viruses, overlapping genes can code for more than one polypeptide.
• Some genes code for a long polyprotein.
• In eukaryotes, genes may contain exons and introns, so they include coding and non-coding regions.

• Genes determine the physical and metabolic characteristics of cells.
• They are responsible for the transmission of characters from one generation to another.
• Genes are situated on chromosomes.
• Each chromosome contains many genes.
• Every gene occupies a fixed position on a chromosome.
• Genes are arranged in a linear order on chromosomes.
• A single gene may occur in different forms called alleles.
• Alleles may show dominant and recessive relationships.
• Genes may undergo a sudden heritable change called a mutation.
• Genes can be transferred from one chromosome to another by crossing over or translocation.
• Before chromosome duplication, genes also undergo self-duplication.
• Genes express themselves through the production of proteins (enzymes) that control cell metabolism.

A gene should perform the following functions:

• It should be able to replicate and pass faithfully to progeny.
• It should be able to undergo occasional mutation, and that change should be inherited stably.
• It should carry all the information necessary to programme the functions of a cell.

• Genes are hereditary DNA units arranged linearly on chromosomes that transmit traits from parents to offspring.
• They code for proteins or RNAs and can contain both coding (exons) and non-coding (introns) regions.
• A gene consists of functional subunits: cistrons (coding), recons (recombination), and mutons (mutation).
• They can self-duplicate, exist in alternate forms (alleles), and undergo heritable changes (mutations).
• Genes control all cellular functions and physical characteristics, primarily through protein synthesis.