Sex-linked characteristics

The first observation of sex-linked characteristics was made using fruit flies. This discovery showed that males and females differ in their chromosome makeup, with females posessing two X chromosomes, and males just one.
 Thomas Hunt Morgan United States 1910 Sex-linked characteristics

The first observation of sex-linked characteristics was made using fruit flies. This discovery showed that males and females differ in their chromosome makeup, with females posessing two X chromosomes, and males just one.