An allele is said to be recessive , when it expresses in

Options

A. Heterozygous condition only
B. `F_1` generation only
C. Homozygous conditions only
D. Both homozygous and heterozygous condition.

Answer

To determine when an allele is said to be recessive, we can follow these steps:

### Step-by-Step Solution:

1. **Definition of Alleles**: Understand that alleles are different forms of a gene that can exist at a specific locus on a chromosome. Each organism inherits two alleles for each gene, one from each parent.

2. **Understanding Dominance and Recessiveness**: Alleles can be dominant or recessive. A dominant allele will express its trait even if only one copy is present (heterozygous condition), while a recessive allele will only express its trait when two copies are present (homozygous condition).

3. **Homozygous Condition**: A recessive allele is expressed in the phenotype only when an organism has two copies of that allele. This condition is known as being homozygous recessive (for example, "tt" for a trait where "T" is the dominant allele and "t" is the recessive allele).

4. **Example**: Consider the example of plant height where "T" represents the allele for tall plants (dominant) and "t" represents the allele for dwarf plants (recessive).
- If a plant has the genotype "TT" (homozygous dominant), it will be tall.
- If a plant has the genotype "Tt" (heterozygous), it will also be tall due to the presence of the dominant allele.
- However, if a plant has the genotype "tt" (homozygous recessive), it will be dwarf, showing the recessive trait.

5. **Conclusion**: Therefore, an allele is said to be recessive when it expresses itself only in the homozygous condition (when both alleles are recessive).

### Final Answer:
An allele is said to be recessive when it expresses in the homozygous condition.