1910

Walter Sutton entered the DNA world by suggesting the Chromosomal Theory. As Sutton started to become interested in genetics after the discovery of Mendal's work in the 1900s he thought the chromosomes as units of inheritance. He did, however, point out and state that there were several alleles that must be in one chromosome and thought of them to be inheritied as a whole unit.
By becoming a studen to C.E. Mclung and working with grasshoppers, he discovered "accessory chomosomes" and that they directly related to the sex. By only working with grasshoppers, Sutton didn't realize that the Y choromosome exsisted then and concluded that we have only 22 chromosomes and that the 'X chromosome' was the deteriming factor of sex.