Which of the following results of Thomas Hunt Morgan's experiments with white-eyed mutant flies was unexpected in light of Mendelian genetics?

• A. In the parental cross, a white-eyed male was crossed with a red-eyed female. The F₁ progeny all had red eyes.
• B. The F₁ flies were crossed with each other, and the F₂ progeny included red-eyed and white-eyed flies in a 3:1 ratio.
• C. The mutant allele (white eyes) was recessive.
• D. All of the listed responses are correct.
• E. Among the F₂ progeny, only males had white eyes. All of the females had red eyes.

Answer: (E)

Verified step by step guidance

1. Understand the basics of Mendelian genetics: In Mendelian inheritance, traits are determined by alleles, which are different forms of a gene. Typically, one allele is dominant and the other is recessive. In a simple Mendelian cross, a 3:1 phenotypic ratio in the F2 generation is expected if the trait is controlled by a single gene with two alleles, one dominant and one recessive.
2. Review the experiment setup: Thomas Hunt Morgan crossed a white-eyed male fly (mutant) with a red-eyed female fly. The white-eyed trait is recessive, meaning that the red-eyed allele is dominant over the white-eyed allele.
3. Analyze the F1 generation: All F1 progeny had red eyes, indicating that the red-eyed allele is dominant. This is consistent with Mendelian genetics, where the dominant trait is expressed in the presence of a recessive allele.
4. Examine the F2 generation results: When the F1 flies were crossed, the F2 generation showed a 3:1 ratio of red-eyed to white-eyed flies. However, the unexpected result was that only males had white eyes, while all females had red eyes.
5. Consider the concept of sex-linked traits: The unexpected result suggests that the white-eyed trait is linked to the sex chromosomes. 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). Since the white-eyed allele is recessive and located on the X chromosome, males (having only one X chromosome) will express the white-eyed trait if they inherit the recessive allele, while females would need two copies of the recessive allele to express the trait.