What is a gene ?

These are 19000 to 20,000 genes in the human body initially scientists believed that humans might have around 1,00,000 genes. But it was found that some genes are repeated. Hence the number of genes is assumed to be around 20,000.
These genes help in production of products which control the functioning of our body and contribute to our physical and mental characters eg. eye color , hair color, height , intelligence.