Drosophila melanogaster: Sex Linked Inheritance Pattern of White Eye Color
Thomas Hunt Morgan used Drosophila (fruit flies) in genetic experiments to support the evidence of the chromosome theory of heredity. Morgan knew that in Drosophila, females have two X chromosomes and males have only one X chromosome. Morgan proposed that the gene for the eye color variant (white) is located on the X chromosome and that there is no equivalent gene on the Y chromosome.
The condition of X-linked genes in the males is said to be hemizygous since there is no corresponding allele on the Y chromosome.