Sex determination by chromosomes in human and Drosophila is through

Options

A. XX-XY
B. XX-XO
C. ZZ-ZW
D. Both A and B

Answer

### Step-by-Step Solution:

1. **Understand the Question**: The question asks about the mechanism of sex determination by chromosomes in humans and Drosophila (fruit flies).

2. **Identify the Chromosomal System**: In humans and Drosophila, sex determination occurs through the XX-XY system. In this system:
- Females have two X chromosomes (XX).
- Males have one X and one Y chromosome (XY).

3. **Evaluate Other Options**:
- The XXXO system is found in certain insects like crickets and grasshoppers, where females have three X chromosomes (XXX) and males have one X chromosome (XO). This is not applicable to humans or Drosophila.
- The ZZ-ZW system is seen in birds, where males are homogametic (ZZ) and females are heterogametic (ZW). This system does not apply to humans or Drosophila either.

4. **Conclude the Correct Answer**: Based on the analysis, the correct chromosomal system for sex determination in humans and Drosophila is the XX-XY system.

### Final Answer:
Sex determination by chromosomes in humans and Drosophila is through the XX-XY system.

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