A recessive allele is expressed in

Options

A. heterozygous condition only
B. homozygous condition only
C. `F_(3)` generation
D. both homozygous and heterozygous conditions

Explanation

**Step-by-Step Solution:**

1. **Understanding Alleles**: Alleles are different forms of a gene that can exist for a particular trait. Each trait is influenced by two alleles, one inherited from each parent.

2. **Dominant vs. Recessive Alleles**:
- A dominant allele is expressed even if only one copy is present (heterozygous condition).
- A recessive allele is only expressed when two copies are present (homozygous condition).

3. **Homozygous Condition**: When an individual has two identical alleles for a trait (e.g., TT or tt), it is referred to as homozygous. The recessive allele (e.g., 't') will only show its effect when in this homozygous state (tt).

4. **Heterozygous Condition**: When an individual has one dominant and one recessive allele (e.g., Tt), the dominant allele (T) will mask the effect of the recessive allele (t), meaning the recessive trait will not be expressed.

5. **Conclusion**: Based on the definitions and examples, a recessive allele is expressed only in the homozygous condition (when both alleles are recessive).

6. **Final Answer**: Therefore, the correct answer to the question "A recessive allele is expressed in" is **homozygous condition only**.

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