Which of the following truly represents a heterozygous organism?

Options

A. Xxyy
B. RRYy
C. XXYY
D. RrYy

Answer

To determine which of the given options truly represents a heterozygous organism, we need to understand the definitions of heterozygous and homozygous.

### Step-by-Step Solution:

1. **Understand Heterozygous vs. Homozygous**:
- A heterozygous organism has two different alleles for a specific gene (e.g., Aa).
- A homozygous organism has two identical alleles for a specific gene (e.g., AA or aa).

2. **Analyze Each Option**:
- **Option 1**: Contains alleles X (big X and small x) and Y (both small y).
- For X: big X and small x indicate heterozygous (Xx).
- For Y: both small y indicate homozygous (yy).
- Conclusion: This option is not truly heterozygous since Y is homozygous.

- **Option 2**: Contains alleles RR and YY.
- For RR: both R indicate homozygous (RR).
- For YY: both Y indicate homozygous (YY).
- Conclusion: This option is not heterozygous.

- **Option 3**: All alleles are homozygous.
- Conclusion: Since all genes are homozygous, this option does not represent a heterozygous organism.

- **Option 4**: Contains alleles that are heterozygous.
- For example, if it has alleles like Aa and Bb, both pairs are heterozygous.
- Conclusion: This option represents a truly heterozygous organism.

3. **Final Conclusion**:
- After analyzing all options, the fourth option is the only one that represents a truly heterozygous organism.