Both alleles express in heterozygote when they are

Options

A. Recessive
B. Lethals
C. Semidominant
D. Codominant

Answer

To solve the question "Both alleles express in heterozygote when they are," we will analyze the options provided: recessive, lethal, semi-dominant, and co-dominant.

### Step-by-Step Solution:

1. **Understanding Alleles**:
- Alleles are different forms of a gene that can exist at a specific locus on a chromosome. In a heterozygous condition, an individual has two different alleles for a particular gene.

2. **Analyzing Recessive Alleles**:
- Recessive alleles do not express themselves in the presence of a dominant allele. Therefore, in a heterozygote (e.g., Aa), the recessive allele (a) is masked by the dominant allele (A).
- **Conclusion**: Recessive alleles do not express in heterozygotes.

3. **Understanding Lethal Alleles**:
- Lethal alleles can lead to the death of an organism if present in a homozygous condition (e.g., AA). However, this does not pertain to the expression of both alleles in a heterozygote.
- **Conclusion**: Lethal alleles do not ensure expression of both alleles in heterozygotes.

4. **Analyzing Semi-Dominant Alleles**:
- Semi-dominant alleles are those that do not fully express themselves when paired with another allele. Instead, they may produce an intermediate phenotype.
- **Conclusion**: Semi-dominant alleles do not fully express both alleles in heterozygotes.

5. **Understanding Co-Dominant Alleles**:
- Co-dominant alleles are different alleles that both express themselves fully in a heterozygous condition. This means that both traits are visible and contribute to the phenotype.
- An example of co-dominance is the ABO blood group system, where both A and B alleles are expressed in individuals with AB blood type.
- **Conclusion**: Co-dominant alleles express both traits in a heterozygote.

6. **Final Answer**:
- Based on the analysis, the correct answer to the question is **co-dominant**.

### Summary:
Both alleles express in a heterozygote when they are **co-dominant**.