A gene is the basic physical and functional unit of heredity. A stretch of DNA which gives instructions to make a specific protein for example insulin (a protein which metabolizes sugars) or melanin (which gives color to our skin and hair) is known as a Gene. Each chromosome has several hundreds to thousand genes
In humans, genes vary in size from a few hundred DNA bases to more than 2 million bases. 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. These small differences contribute to a person’s unique physical features, susceptibility to disease and response to therapy.
A change or alteration in any part of the DNA which makes a gene is known as a Mutation.
Hereditary mutations are inherited from a parent and are present throughout a person’s life in virtually every cell in the body. These mutations are also called germline mutations because they are passed on from the parent’s egg or sperm cells, which are also called germ cells.
Acquired (or somatic) mutations occur at some time during a person’s life and are present only in certain cells, not in every cell in the body and cannot be passed on to the next generation. Some of these are responsible for diseases like cancer.