A gene is the basic physical and functional unit of heredity. Genes are made up of DNA. Some genes act as instructions to make molecules called proteins. However, many genes do not code for proteins. In humans, genes vary in size from a few hundred DNA bases to more than 2 million bases.
A gene is a nested sequence of nucleotides in DNA or RNA that encodes the synthesis of a gene product, either RNA or protein.
Lewin (2000) has defined gene to be a sequence of DNA that codes for a diffusible product.
A gene is:
(i) A unit of genetic material which is able to replicate,
(ii) It is a unit of recombination, i.e., capable of undergoing crossing over,
(iii) A unit of genetic material which can undergo mutation,
(iv) A unit of heredity connected with somatic structure or function that leads to a phenotypic expression.
The Human Genome Project estimated that humans have between 20,000 and 25,000 genes.
Every person has two copies of each gene, one inherited from each parent. Most genes are the same in all people, but a small number of genes (less than 1 percent of the total) are slightly different between people. Alleles are forms of the same gene with small differences in their sequence of DNA bases. These small differences contribute to each person’s unique physical features.
example, a gene on chromosome 7 that has been associated with cystic fibrosis is called the cystic fibrosis transmembrane conductance regulator; its symbol is CFTR.
Functions of Genes
- Genes control the functions of DNA and RNA.
- Genes are components of genetic material and are thus units of inheritance
- They control the morphology or phenotype of individuals
- Replication of genes is essential for cell division
- Genes carry the hereditary information from one generation to the next
- They control the structure and metabolism of the body
- Reshuffling of genes at the time of sexual reproduction produces variations
- Different linkages are produced due to crossing over
- Genes undergo mutations and change their expression
- New genes and consequently new traits develop due to reshuffling of exons and introns
- Genes change their expression due to position effect and transposons
- Differentiation or formation of different types of cells, tissues and organs in various parts of the body is controlled by expression of certain genes and non-expression of others
- Development or production of different stages in the life history is controlled by genes
- Gene produces proteins through the process of gene expression( transcription, translation). Proteins are the most important materials in the human body which not only help by being the building blocks for muscles, connecting tissue and skin but also takes care of the enzymes production.
- These enzymes play an important role in conducting various chemical processes and reactions within the body. Therefore, protein synthesis is responsible for all activities carried on by the body and are mainly controlled by the genes.
- Genes consist of a particular set of instructions or specific functions. For example, globin gene was instructed to produce haemoglobin. Haemoglobin is a protein that helps to carry oxygen in the blood.