Gene is a specific segment of DNA molecule which determines the inheritance and expression of a particular character.
It is the functional unit of heredity which carries information from one generation to the next.
Two or more alternative forms of a gene are called alleles or allelomorphs. They occupy the same gene locus on homologous chromosomes but produce different effects on the phenotypes.
In pea, gene for producing height of plant may occur in two alternative forms: Tall(T) and Dwarf(t). Tall and Dwarf forms of gene are alleles of each other.