Form a small discussion group and decide on the most likely genetic explanation for each of the following situations;

Cross A performed by Morgan and shown in Figure 3.18 is between a mutant male fruit fly with white eyes and a female fruit fly from a pure-breeding, red-eye stock. The figure shows that 1237 F₁ progeny were produced, all of them with red eyes. In reality, this isn't entirely true. Among the 1237 F₁ progeny were 3 male flies with white eyes. Give two possible explanations for the appearance of these white-eyed males.

Key Concepts

Sex-linked Inheritance

Sex-linked inheritance refers to genes located on sex chromosomes, particularly the X chromosome in many organisms, including fruit flies. In this case, the white-eye trait in Drosophila melanogaster is known to be X-linked. Since males have one X chromosome, a single recessive allele for white eyes can manifest in male offspring, explaining the appearance of white-eyed males among the F₁ progeny.

Mutations and Genetic Variation

Mutations are changes in the DNA sequence that can lead to new traits or variations within a population. In the context of the cross described, the appearance of white-eyed males could be due to spontaneous mutations occurring in the F₁ generation, resulting in a new allele that expresses the white-eye phenotype. This highlights the role of genetic variation in evolutionary processes.

Incomplete Dominance and Epistasis

Incomplete dominance occurs when the phenotype of heterozygotes is intermediate between those of the homozygotes, while epistasis involves the interaction of different genes where one gene can mask the expression of another. In this scenario, if the white-eye trait is influenced by multiple genes, it is possible that an epistatic interaction could lead to the expression of white eyes in some males, despite the presence of the red-eye allele.