Morgan's work on Drosphila melanogaster proved that genes are located on the chromosomes. He crossed red eyed female with white eyed male. All the individuals were red eyed. On inbreeding, all the $F_2$ females were found to be red eyed, 50% of the males were red eyed while the remaining 50% $F_2$ males were white eyed.
Red eyed $F_1$ females were crossed with white eyed males. It yielded red eyed females, white eyed females, red eyed males and white eyed males in the ratio 1:1:1:1.
When white eyed females were crossed with red eyed males, all the $F_1$ males were white eyed and all the $F_1$ females were red eyed.
Cytological studies showed that females have XX chromosomes while the males have XY chromosomes where only X is linked with eye colour, as males pass the X - chromosome to female offspring, it is criss cross inheritance which proves that genes are located on the X - chromosome.