A cross of white eyed female and red eyed male drosophila gives red eyed females and white eyed males. Rarely this cross may give all white eyed females and red eyed males. This was found to be due to

Options

A. loss of a sex chromosome
B. non disjunction of X chromosomes in female
C. mutation in female fly
D. mutation in male fly

Answer

To solve the question regarding the cross of a white-eyed female and a red-eyed male Drosophila, we need to analyze the genetic inheritance of eye color in these organisms. Here’s a step-by-step breakdown of the solution:

### Step 1: Understand the Genetic Basis
- In Drosophila, the eye color trait is sex-linked and is located on the X chromosome. The red eye color (R) is dominant over the white eye color (r).
- Females have two X chromosomes (XX), while males have one X and one Y chromosome (XY).

### Step 2: Determine the Genotypes of the Parents
- The white-eyed female has the genotype X^rX^r (both X chromosomes carry the recessive allele for white eyes).
- The red-eyed male has the genotype X^RY (the X chromosome carries the dominant allele for red eyes).

### Step 3: Perform the Cross
- When we cross the white-eyed female (X^rX^r) with the red-eyed male (X^RY), we can predict the offspring:
- The female offspring will inherit one X chromosome from each parent, resulting in X^RX^r (red-eyed females).
- The male offspring will inherit the X chromosome from the mother and the Y chromosome from the father, resulting in X^rY (white-eyed males).

### Step 4: Analyze the Rare Occurrence
- The question mentions a rare occurrence where all offspring are white-eyed females and red-eyed males. This can happen due to a genetic phenomenon known as non-disjunction.
- Non-disjunction occurs when the X chromosomes fail to separate properly during meiosis in the female, leading to gametes that may have two X chromosomes or none.

### Step 5: Determine the Resulting Genotypes
- If non-disjunction occurs, it could lead to a female with the genotype X^rX^r (white-eyed) and a male with the genotype X^RY (red-eyed).
- This results in all offspring being white-eyed females (X^rX^r) and red-eyed males (X^RY).

### Conclusion
- The rare occurrence of all white-eyed females and red-eyed males from the cross of a white-eyed female and a red-eyed male Drosophila is due to non-disjunction of the X chromosome during gamete formation in the female.

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