In previous topics, various principles of genetics were explained with the help of a unit of heredity, the gene. Although the term genetics was first used by Bateson in 1905, the term gene (derived from the Greek word genetics, meaning to be born) was proposed in 1909 for the first time by Johannsen. It was subsequently also regarded- as a unit of function, unit of mutation and unit of recombination. This means that a gene was believed to be a hereditary unit, which was independent in controlling a function leading to appearance of a phenotypic trait for which it was meant. It was also the smallest unit that could undergo a mutational change or could take part in recombination. Such a view was the result of a belief in particulate inheritance, according to which a gem; was a locus or a point having no dimensions.

In this section, we will discuss the work, which led to changes in our concept of allelomorphs and allowed a study of fine structure of gene at the intragenic level. It will be shown how micromaps for individual genes could be prepared through recombination technique.