The concept of heredity arose when the ancient philosophers and scientists felt the need to explain the variation and organic evolution phenomena. The ideas about inheritance developed before Mendel were significant in the construction of the Mendelian concept of gene. From Mendelian hereditary principles to molecular genetics there have been many different concepts and also many definitions of gene. In the first corpuscular concept of gene, mutation was quite crucial to explain the different alternative genotype and phenotype expression in the progeny. From the rediscovery of Mendelian Principles to 1961, Morgan's idea that a gene is not divisible by recombination prevailed. Nevertheless it was later demonstrated that there are different units of recombination and mutation within the gene, and in a determinate gene different "functional units" can exist. In 1977, surprisingly, Sharp and Roberts found out that genes are fragmented into "exons" and "introns". At present time, with the discovery of iRNA, non coding RNA, importance of introns, transposable elements, pseudogenes, endogenous viral DNA, repeated DNA, superposed genes, non-transcriptional genes and epigenesis, ancient questions return: what is a gene? where is the program? what is the true role of mutations in the organic evolution?