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In the most basic terms, genome is the genetic material of an organism. It is what tells the organism what to be and how to be. The genome is coded in DNA, or in RNA for many types of viruses. The term, genome, was coined in the 1920, as a portmanteau of gene and chromosome.
What is a gene? A gene is the basic physical and functional unit of heredity. Genes, which are made up of DNA, act as instructions to make molecules called proteins. In humans, genes vary in size from a few hundred DNA bases to more than 2 million bases. The Human Genome Project has 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.