Question

when thomas hunt morgan crossed his red - eyed f1 generation flies to each other, the f2 generation included both red - and white - eyed flies. remarkably, all the white - eyed flies were male. what was the explanation for this result?
the gene involved is on the y chromosome.
eye color in flies is a multifactorial trait.
the gene involved is on an autosome, but only in males.
the gene involved is on the x chromosome.

Explanation:

Thomas Hunt Morgan's experiments with fruit flies (Drosophila) led to the discovery of sex - linked inheritance. In Drosophila, the gene for eye color is located on the X chromosome. Males have one X and one Y chromosome ($XY$), while females have two X chromosomes ($XX$). When Morgan crossed red - eyed $F_1$ generation flies, the $F_2$ generation had white - eyed males. This is because males only need one copy of the recessive white - eye allele (on their single X chromosome) to show the white - eye phenotype, while females need two copies (one on each X chromosome). If the gene were on an autosome, the phenotypic ratio would not show such a strong sex - bias. Multifactorial traits are influenced by multiple genes and the environment, which is not the case here. The Y chromosome does not carry the eye - color gene in Drosophila.

Answer:

The gene involved is on the X chromosome.