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 the figure below, 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.

Verified step by step guidance

1. Step 1: Understand the genetic basis of eye color in fruit flies as studied by Morgan. The white eye mutation is X-linked recessive, meaning the gene responsible is located on the X chromosome, and the white-eye phenotype appears in males if they inherit the mutant X chromosome, since males have only one X chromosome (XY).
2. Step 2: Analyze the cross described: a mutant white-eyed male (X^w Y) crossed with a pure-breeding red-eyed female (X^+ X^+). The expected F1 progeny should all have red eyes because females provide the dominant red-eye allele (X^+), and males inherit the X^+ from their mother, resulting in red eyes.
3. Step 3: Consider why 3 white-eyed males appeared among 1237 F1 progeny. One possible explanation is a rare event of nondisjunction during meiosis in the female parent, leading to sperm or eggs with abnormal sex chromosome numbers, which could produce males with the mutant X chromosome.
4. Step 4: Another possible explanation is a spontaneous mutation occurring in the X chromosome of some F1 males, converting the red-eye allele to the white-eye allele, resulting in white-eyed males despite the expected genotype.
5. Step 5: Summarize that these explanations involve either chromosomal abnormalities (nondisjunction) or new mutations, both of which can cause unexpected phenotypes in genetic crosses, especially when dealing with X-linked traits.

Key Concepts

Sex-Linked Inheritance

Sex-linked inheritance refers to genes located on sex chromosomes, often the X chromosome in fruit flies. Since males have one X and one Y chromosome, recessive mutations on the X chromosome, like the white-eye mutation, are more likely to be expressed in males. This explains why white eyes often appear in male progeny when inherited from carrier females.

Mutation and Reversion

Mutations are changes in the DNA sequence that can alter gene function. Occasionally, a rare reversion mutation can restore the original gene function, causing unexpected phenotypes. In this case, a few white-eyed males might arise due to new mutations or reversions affecting the eye color gene.

Nondisjunction and Chromosomal Abnormalities

Nondisjunction is the failure of chromosomes to separate properly during meiosis, leading to gametes with abnormal chromosome numbers. This can produce males with unusual combinations of sex chromosomes, such as XXY or XO, potentially causing unexpected expression of X-linked traits like white eyes in males.