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 x 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 y chromosome.

Explanation:

In Drosophila (fruit flies), sex is determined by the X and Y chromosomes. Males have the XY chromosome combination and females have the XX combination. If a gene is located on the X chromosome (X - linked), males only have one copy of that gene (since they have only one X chromosome). In Morgan's experiment, the white - eyed trait was only seen in males. If the gene for eye color was on the X chromosome, a recessive allele for white eyes on the X chromosome in males (who have only one X) would be expressed. Females (with two Xs) would need two copies of the recessive allele to show the white - eyed phenotype.

• For the option “Eye color in flies is a multifactorial trait”: A multifactorial trait is influenced by multiple genes and environmental factors. But the pattern of only males showing the white - eyed trait is more consistent with X - linked inheritance rather than a multifactorial trait.
• For the option “The gene involved is on an autosome, but only in males”: Autosomes are non - sex chromosomes. If the gene was on an autosome, the trait would not show a sex - specific pattern (all white - eyed flies being male) in the F2 generation.
• For the option “The gene involved is on the Y chromosome”: If the gene was on the Y chromosome, only male offspring would inherit it. But in Morgan's cross, the F1 generation had red - eyed flies (both male and female). If the white - eyed trait was Y - linked, all male offspring would have white eyes, which was not the case.

Answer:

The gene involved is on the X chromosome.