Final answer:
Thomas Hunt Morgan mapped the gene for eye color in Drosophila to the X chromosome in 1910, demonstrating that red eye color is dominant and that males are hemizygous for this X-linked trait.
Step-by-step explanation:
Thomas Hunt Morgan made a significant contribution to the field of genetics by discovering that the gene determining the eye color in Drosophila melanogaster, the common fruit fly, is located on the X chromosome. In 1910, Morgan's research showed that red eye color (XW) is dominant compared to white eye color (Xw) and that the X-linked traits were inherited differently in males and females due to their chromosome composition (XY in males and XX in females).
Morgan observed the nature of heredity by studying the inheritance patterns and found that males are hemizygous for X-linked traits, meaning they carry only one allele for traits on the X chromosome, making the descriptions of dominance and recessivism not applicable to XY males. Drosophila males have genotypes of either XWY or XwY while females can have genotypes of XWXW, XWXw, or XwXw, which determine the eye color.